There's no really easy way to do this on the server side, given the current implementation. One possibility would be to encrypt the lists, then decrypt them on the client side, and let the client do the filtering. But it would be a big change.

But this could make more sense in terms of some of the things I'm planning for the reputation system, and we might be able to use that for filtering (again, it would imply the filtering is done on the client side, instead of hivemind).