I usually try to create a new thread if the situation warrants it. I have forums for everything. If it's someone new breaking the rules I'll refer them to the TOS. Newbies never read the rules. If it's just someone being a douchebag I'll hard delete the post(s) and send them ONE warning. I have no issue permanently banning someone who insists upon ruining the forums for the rest of the community. Nobody has time to babysit the forums.
Split the thread, posting a link to the new one from the old. May issue a polite warning. For repeat offenders, gradually escalate to infractions of increasing magnitude.
I split the thread, make both threads irrelevant of each other, and warn the user. If it is a subject already covered, I'll merge the offending post to a "Don't post in the wrong places" thread. Then explain what's going on. Sometimes I'd PM the user explaining not to do this or that. If it's a repeat offender, I deal with them on a case by case basis. 3 strikes or you're out. It's that simple.
Same as this i would delete the posts which i consider hijacks and then offer out warnings for the members who did it, i'm not strict but i can be in situations like this.
I just politely ask people to write another thread. If they don't, I give them a polite warning and take further action if required.
If it gets too much off-topic from the original thread title, then I'll move the posts to its own thread and go from there. It's better to split the discussions because you can probably yield more content from the users when you do this.
I tend to do nothing. I let the thread take its own direction. Isn't that sort of the point of discussions? To see what sort of digressions the conversation will take? The topics that conversations lead to are often the "meat of the matter". This isn't always the same as the exact topic that originated the conversation. There is nothing wrong with digressions. If the forum members want the topic to resume its original intent then it will happen naturally.
This is true and I've seen this happen a few times. Sometimes members get off topic because they're trying to prove a point or discuss something similar to the thread at hand, then they'll stir the discussion back to the main topic after a few posts of being off topic. Members love doing this type of stuff because it also generates more content for your forum.