That's an incredibly subjective benchmark, especially on a forum whose first principal is free speech.

A lot of possible solutions require a fair bit of active moderation.  Splitting derailed threads works well of you can do it quickly enough.  You can split and lock the derailment and send it to some wasteland forum, but you need to be able to catch the derailment when it first happens otherwise it's a pain in the ass to do it manually once you've got a couple of pages of legitimate posts interspersed with derailing posts.