Basically, take Dark0ne's advice, aspire to be just like Peregrine was... Wait... No... That could be bad...
I'd like to point people to the "Report Post To A Moderator" feature. I check my email daily so reported posts will be checked up on. The only member known to actually use this feature effectively is Peregrine *claps*
Perhaps this second bit is what you should walk away with...
Basically, it is not your job to tell people what they should do, they should know what to do on their own. When they don't, then it becomes the job of staff to remind them.
So, when you see an example of thread necromancy, spamming or wrong doing don't start replying to the thread with reasons why it should be deleted et cetera, because more than likely your post will be deleted too and you'll be affected by the same -4 post count deducted the spammers get.
So, conclusion: Use report moderator feature, don't start telling people what they can and can't do. Thats our job.
In chat, since all discussion is immediate, and often makes other conversations hard to keep up, reminding people that they should change the discussion before it goes somewhere bad is a good idea, but should usually be phrased as a suggestion.
The problem comes when people take their role of reminding others too far, or do it in a way that it looks like they're trying to suck up to the moderator in the room, or the staff who might handle the case.