Ways to block Lemmy.ml:

1. on base Lemmy, you basically cannot - for whatever reason, the devs do not seem to consider this a “priority”?:-P
2. the Sync or Connect apps let you do this though
3. PieFed allows you to block all the users from any instance that you choose, without needing admin support; PieFed also has fantastic features such as hashtags and categories of communities, though notably its base UI is significantly less advanced than Lemmy’s, e.g. it lacks user tagging (e.g. @openstars@piefed.social does nothing)
4. the only instances that have defederated from the big 3 are lemmy.cafe and dubvee.org (also quokk.au but that has a single admin and people report not being able to join it)

More details in this post.