On my boards I am admin on, my procedures would be:
1) Start a new thread titled something like "Lawlz/Thomas argument"
2) post a link back to the thread it was split from
3) move all derailed posts to that new thread
4) post in this thread that the argument was moved to the new location.
...Now the thread should still be on track and they can bicker over who contributes more to the forums.
A point of clarification, Chris.
That sounds really good if that were to happen, but in your scenario, who has the power to do all of that? Up until your post at the top of page 4 here, it sounded like you, as the thread creator would have that power. But then you said you are a moderator also. Would whomever creates the thread have the power to manage it, or just the moderators? Taking that question further, it sounds like you would manage your threads responsibly, but what would happen if these threads were created by less responsible people who would just get rid of opposing viewpoints?
Another suggestion for improvement: I like to know what I'm talking about before I post something, so I frequently need to go back a page or two while I'm writing. To do this, I have to copy onto the clipboard whatever I've written then paste it back when I'm ready to write again. Could we make it so we could change pages and not lose what we've written.