How to stop incoming updates?

Since I experimented with ActivityPub my server keeps getting updates about user deletions from other instances like mastodon.social

How can I stop those?

Not really, to ensure accounts are deleted across the fediverse, Mastodon sends Delete on Actors to all the inboxes (or shared inboxes) it knows. It’s the only kind of updates Mastodon currently sends when there are no follow relationships, though.