It's a tough spot to be in, having tried to coach them towards what would work. They might find being a martial class if they see great success in it. Perhaps a one shot where the players are all some martial build, to show how successful their playstyle would be with the right tools.

Another idea is to suggest classes like Monk or a psywarrior which can use "magic" but are more durable to hits and can skip their powers without losing 80% of their contribution to the fight.

Finally you can lean into it and have them play characters that are conscientious objectors to magic or are scared of it but they have a gift/been railroaded by their guardians. The party can adapt and roleplay around it and maybe encourage the player (in character no less) to use their abilities.

Hopefully one of these helps!

“I run up and stab it with my dagger!”

“Are you sure? As a wizard, your dagger is very ineffective and puts you in harm’s way. You could cast fire bolt from where you are standing. You’d have a better chance to hit, do more damage, remain safe, and play to your character’s strengths more. Do you want to do that instead?”

“I’m trying to save my spells for an emergency”

“Well fire bolt is a cantrip, so it never runs out and you can use it every turn like a fighter would use their weapon. Cantrips are the ‘auto-attacks’ for spell casters”

I can’t understand your situation OP if the exchange I described above isn’t the solution. I play with newbies and first timers all the time and we constantly strategize in combat so they can learn how to play as we go. Would your player really say “no, I don’t care, I stab them” after being presented with that option? If so, I think they are doing this intentionally because they think it’s funny or interesting, not because they don’t know better

Is the player choosing a caster for "cosmetic" reasons, or because they don't know better (like recycling the last character sheet)?

If the first, then maybe they could play a character who disguises as caster, or something along the lines.

If it's the latter, then they would probably appreciate it if you helped them create a new character from scratch together.

The solution is simple: Remove the dagger mid-combat.

You could make the dagger too hot to hold and it falls out of reach (off a mountain, into rushing water, etc...)

You could make the dagger dissolve away (through lava, acid, being eaten, etc...)

You could make something take the dagger (disarming, stealing, etc...)

A hag/genie/etc could place a curse on the PC (holding anything makes them experience immense pain and drop what they're holding, anything dagger they hold is turned into a spoon, etc...)

Lots of good advice here already, but I would also suggest this: if they like playing a spellcaster but aren't very familiar with the spells available, spell cards could be very helpful. If you don't want to buy official ones, there are sites where you can print your own. I did this for one of our players and it helped her a ton.

Could they play a Hexblade warlock?

How about making a character that fakes being a caster, and like treats everybody like they don't deserve to see his magic meanwhile he's actually a rogue? Looks and behaves like a caster, is just still useful in combat

You're obviously aware of their playstyle, and if they aren't very well versed in the rules, maybe you could roll a couple of melee focused spellcaster (Hexblade Warlock, Bladesinger Wizard, etc) options for them and have them choose from one of them rather than relying on them to come up with something on their own? They'd still have some agency, but would play a character that better suits their playstyle.

Oh, and make sure the options don't have a dagger in their equipment list. 😄

The player obviously is stubborn and won’t apply what you’ve told them.

Kill their character. Their wizard with high intelligence wants to melee? The monsters are smart and they see the robe guy who gets stabbed easily - so that’s what they do. All. Of. Them.

Kill, kill, kill. Then have a chat with them after about why this happened. Let them know they know how to play the game but actions have consequences and the monsters are smart. You’ve let them have their learning curve and now there are stakes.

If Player is receptive to directions in the moment, perhaps a queue card would help.

Stuff like;

Positioning: As a caster, you are easy to hit and have relatively low HP. Try to stay 15-30ft away from the nearest enemy. -Relevant Abilities:

• • 30ft Movement
• • Disengage (1 x Action)
• • Misty Step (1 x Bonus Action, 1 x level 2 Spell Slot)

Damage:

Healing:

Support:

Play hardball, they're squishy, break some bones if they don't use their spells and run at things I'n cloth instead of plate.

They break their fingers and can no longer hold a dagger. Luckily this injury doesn't impact their spellcasting abilities.

Also I want to say that it would be normal for most big baddies to laugh at a 10 hp robe wearing mage poking them to ignore them and laugh like a 5 yr old kicking at their feet. They maybe don't have to smite them immediately.

Why would a player drawn to spellcasters prefer weapon attacks over cantrips? I think that's the question we need to focus on.

Are they having trouble finding spells that fit the theme of their character? If so, maybe they'd benefit from looking at a wider range of source books. You can also remind them that they can "reflavor" their spells to better fit their character.

Is it utility magic that draws them to spellcasters? Are they even choosing offensive spells to begin with? If not, maybe you can help them out with a magic item - something to guarantee that they have access to a spell or two that would benefit the party. You could also shift some of the danger towards non-combat encounters to give their spells more value. For what it's worth I don't think there's anything wrong with utility-focused characters - they just tend not to shine as much in one-shots. Maybe if they knew more about the adventure ahead of time, they could better cater their character to the situations they're likely to face.

Are they overwhelmed by their character sheet? If so, playing short campaigns starting from level 1 might help to ease them into their class abilities. Spell cards might also be helpful if you're playing tabletop.

Appropriate: https://youtu.be/vr_FPbAHids?si=sLBGiN0IShtyseoS

This sounds like someone that doesn't take the time to read and understand the mechanics of their chosen class or the spells and how they work. That's unfortunate.

Have you discussed how their actions are impacting the other players, and that their play is leading to the deaths of their friends? They may not realize how irritating it can be from the friends' point of view.

It seems as though they want a wizard-warrior, almost like a Jedi--who fights with swords, but has magic spells too. There are many ways to achieve this character idea and I'm sure you're more aware of them than I, but it could be as simple as a fighter with the magic adept feat.