I think filtering on the relay side will still work better than on the client.
With that said, using reactions to signal stuff to to relays might be a good idea.
Discussion
👍🏻 I like idea of using kind=7 reactions to encode all sort of “reactions” to relays including flagging. Someone will need to maintain a registry of kind 7 reactions just as now we need a registry of kinds (I think I saw someone organizing this already)