Allow Mods to Lock Specific Users Out of Specific Threads

Concise description:
Specifics:
This will improve the following aspects of the site:
- Sometimes threads that have constructive discussions ongoing are locked by mods b/c community members get into pissing matches with each other. Many times the flaming is gracefully dodged by other members and discussion continues. If the pissing match goes on long enough a mod comes and locks down the topic (rightfully so). The issue is that the ongoing discussion is mowed down as an innocent bystander in this locking drive-by. The Maxatstuy thread is an excellent example.
Specifics:
- Give the mods the ability to lock specific users out of specific threads. (Functions like a targetted forum ban.)
- Mods maintain their current ability to completely lock a thread.
This will improve the following aspects of the site:
- Threads are not prematurely locked.
- On-topic discussion can continue where it began instead of being shut down.
- Discourages flaming outside appropriate venues.