I think Lemmy 0.19 has this feature, though if you're having problems with spam users from those instances you'll need to report them or have your administrator deal with them, as the domain blocking feature does not hide users.

Also even if it did the act of hiding users isn't great since people commiting violations need to be dealt with or otherwise impeded from doing so, and blocking doesn't stop them whatsoever. At least on Reddit it locked them out from replies but here it's the same as ignoring them. I know people didn't like that but the majority of people who I've seen complain about being blocked on Reddit (especially by me) were:

1. Transphobes, truscum included (I blocked a lot of them when the blocking changed to Two way)
2. Trolls (especially the alt-right variety, but a decent amount of commies, tankies, and your average "enlightened centrists")
3. Scammers
4. Pedophiles (AKA "MAPs")