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.
The development of UAD has been stopped/paused. The fork Universal Android Debloater Next Generation continues it