Currently: Not for a user.
The only way to block at instance level is defederation which is usually a bit extreme.
This shouldn't be too complicated to handle at the user level, but someone would have to spend time to develop it and contribute to lemmy source code.
Likewise, there are specific instances for which I'd want to auto-sub to all current and future communities, which is also not a fair that exists (other than creating your account there and browsing "Local")
IMHO, Blocking at the user level is a feature that's relatively likely to happen eventually.
Out of curiosity how intensive is it to run a lemmy instance just for your own personal use?
Kinda curios if federation content from large instances (like lemmy.world and lemmy.ml) and serving pictrs hosted on your personal to said larger instances is resource intensive (bandwidth/cpu).