we are legion, you can never block us all

The best way is to make an account on a server that doesn’t federate with them. Sh.itjust.works, lemmy.world, and most other large servers are defederated from hexbear and lemmygrad.ml.

Apparently lemmy.cafe is defederated from those as well as lemmy.ml. Be aware that a good chunk of major communities are hosted in lemmy.ml, so you won’t be able to access those if you’re defederated.

Unfortunately even if you block them, you will still see posts and comments that users from that server make on external communities. It’s only by being fully defederated that you won’t be able to see any of their content at all.