Personal blacklists for other players.

[CortexReaver]

All text messages from a person in such list will be blocked. Blocking by IP and registered accounts. Blacklisted player needs some sort of an indication for being muted, like an icon, and going further with this idea such player's nickname can also be replaced with something like " muted player ". So he cannot communicate with you through nickname changing (streamers be especially grateful for this).
Best place for the list itself will be a separated tab in options menu.