Almost everyone who interacts with any kind of wizard would be extremely guarded around giving consent for anything since it might just be a ploy to remove their resistances.

And that's totally fair and matches a lot of wizarding canon. It could very well be that this NPC isn't particularly bright, or at least not accustomed to dealing with wizards, but the DM can come up with some clever way to still have the story progress (i.e. the NPC happened to be wearing an amulet that protects them from magic, the NPC can communicate telepathically when transformed (so the story can continue), the magical power necessary knocks out the wizard and the spell link is broken, or the transmutation on an unwilling human is temporary and the wizard needs to roll X times above Y to maintain the spell (and X gets lower as the NPC submits).

There are a lot of ways to mitigate the impact of an outrageous player choice and discourage them from pulling further shenanigans. Just saying "no" is rarely the most fun option.