Proposal: Add a user access control level:

Mute: You don't see <user>'s posts.
Block: The above, plus <user> can't see your posts.
Damnatio Memoriae: The above, plus you don't see any posts that include <user>'s username. You get a 404 when trying to view <user>'s profile. <user> gets a 404 when trying to view your profile.