Allow a username blacklist.
If Twitch won't filter (or ban) usernames containing personal information, perhaps streamers could blacklist names containing certain words/#s/etc.
I second this. A streamer I mod for (and several others who stream the same game) has been relentlessly harassed by the same troll. He's been at it for nearly 2 years now, and has created well over 1000 alts (it's always the same username followed by a random string of numbers).