"...nobody ever challenges them for it."

My impression is that a main problem is that people often do challenge or respond to hecklers, when it might be better just ignore to them.

Pulling a thread back to a specific aspect is tricky verging on the impossible - one way would be to start a new thread, and post a link to it in the thread that has drifted off into other areas, with an explanation. And maybe supplement this with PMs to any people on the original thread who have shown they were interested in that aspect of the discussion, alerting them to the new thread (because quite likely they might have stopped reading the old one.)

That's more or less the way we do things in ordinary life - if a general discussion veers off somewhere away from what you are especially concerned about, you are likely to find yourself carrying on the original conversation with a couple of people who are interested in the same aspect as you are. You wouldn't go thumping the table and calling the meeting to order, unless it was a formal meeting. And threads here aren't formal meetings.