Hi Mark,
I ran a phpBB forum so I my suggestion comes with experience. I understand there may be times for a moderator to interact, but locking a topic that was not started by a couple guys having a harmless verbal fistacuff hardly seems fair to the topic starter or the dialog that has transpired.
Why not add another forum like Talk About It, Lobby Games, Status, etc. and call it The Pub. By doing this a moderator can move dialog there or tell the guys involved to take it to the Pub. By doing this players can blow off a little steam and not interfere with main stream dialog. You could even put red text under The Pub that reads Enter At Your Own Risk. At least give it some thought it's very easy to maintain and may even provide some entertainment.
Most of the time limited audience is the best way to end an argument.