Currently the admins have to curate this for you through federation, although you can try and whack a mole individual communities from an instance. Heaps of people are asking for user level control of blocking instances and I hope it comes soon as there's a couple instances I keep seeing federated into my feed that I find abhorrent, and this growth phase of Lemmy means new communities on those instances keep appearing.
Oh I didn't mean blocking users, to me a user's home instance is a matter of convenience and not a cross for them to bear as a member of some perceived village.