No, that is not possible, each server (instance, like Lemmy.ml, lemme.ee, Lemmy.world) is completely on its own. The reason you can see posts from instance x on instance y is because they sync posts and comments with eachother. The are what is called “federated”. Some instances unfederate other instances. Then the sync is only one way. Basically saying “I don’t want to see your posts and comments anymore instances x, I will no longer sync your stuff to my instance”. Note that other instances can still sync posts and comments from the one that unfederates. Take for example that beehaw.org unfederated lemmy.world. Users from Lemmy.world can still post on beehaw.org communities but beehaw will not see those posts anymore.