They don't lock forum posts, they lock threads. This is because they are pointless, inappropriate or old. (Not usually old, but sometimes when people necro them.)
They also do it because they will turn into a spam topic.
They don't lock forum posts, they lock threads. This is because they are pointless, inappropriate or old.
That sorta counts as spam.
Anyways, going into further detail; A mod can lock a thread at any time, and if they see a thread that is inappropriate, pointless, spammy, a flaming thread, spam again, or more flaming they will lock it. It is to make sure that; No users can get more posts off it (mainly for spam threads) and that the flaming or argument doesn't continue.