If they aren't mods, then they can't really. There's only so much a community can do without mod abilities. Can't stop spam, can't ban spammers etc.

Depending on the instance and the community in general. It's possible for admins to appoint a new moderator and remove the old ones. Lemmy.world will. I know because one of the communities I moderate was effectively moderatorless for a couple of years. But a few of us in the community stepped up.