The federation between mastodon and lemmy is strange. If a M account wants to follow a L community, they need to follow an automated M account which represents the L community. But if any M post mentions that L community, the post will get boosted by the community’s M account, so everybody who follows will get a notification. And I’m not sure if this can be moderated from the L side, because it seems like it never goes through L. Such as - do you see this @opensource ? Does a L mod see this?
I don’t know what options there are for admins. lemmy.ml could (temporarily) defederate the instances from which the spam comes. From all Mastodon instances that have a completely free registration, at the moment spam is distributed through the whole fediverse. Lemmy users can block individual accounts, but I don’t think this is possible with a whole instance.