Times when it is OK for a thread to go off-topic-When it has served it's purpose - the question has been answered, or the issue was resolved.
-Times when it is NOT OK-When the question has not been answered yet to the best of the community's ability.
-When the question HAS been answered, but the thread is continuing in a meaningful way (eg. A discussion concerning the topic).
How to Deal with someone taking a thread off-topic (eg. Posting a non-related image, saying something funny but not-related, Or posting any content not related, relevant, or useful)-Ignore it. Simply that.
-Allow for *ONE PERSON MAX* to tell the poster that this is inappropriate content, but only if this is absolutely necessary, as this should typically be left for an admin.
How NOT to deal with someone taking a thread off topic-Starting an argument with the person themselves, or having multiply people shout at/discourage this person. In doing so, you are bringing the thread off topic yourself.
-Continuing with this trend and posting something off-topic yourself. This is a terrible thing to do as you are ruining someone's perfectly good thread, and they may not get their question answered or their topic fulfilled, and it makes the thread harder to continue and breaks it up with spam. Nobody who legitimately wants to read a thread wants to skip over bits of spam all over the place.
Now, this is a complete waste of a post, because, well, it's just fucking common sense. You'd think everybody on here without a specific condition would be able to follow this pretty easily.
I'm just writing this up because it's late, I'm not tired, I'm bored of L4D, and feel like doing something constructive.
Ps. I was gonna post this on Ror's thread, but, well, I'd be breaking rule number 1. 
Edit: Use this at your own discretion. Meaning, I know most of your aren't idiots, and know when you can post random funny shit and when you can't, because I want these forums to be useful yes, but not necessarily we-love-the-rulebook-you're-gonna-get-banned-for-posting-anything-fun useful. That is all.