You can communicate with beehaw and other instances, thats the good thing with the Federation! Also you can block comminities, so I would just do that.
Lemmy.world is defederated with Beehaw. That means we can't see their new content and they can't see ours. However, we still have "ghost" versions of the Beehaw communities. Content there cannot synch. I think OP wants to block all of Beehaw so that the "ghosts" can disappear.