Is there such a thing as an #ActivityPub spamfilter?

The #Fediverse already has a very heterogeneous software landscape. I'm reading about proposals to add support to #GitLab as well. But one problem would be common to every implementation: Spam.

And just like MTAs don't each implement their own spam filtering and instead rely on rspamd and spamassassin, I feel like some middleware should exist for ActivityPub as well.