You can remove a post you made in reply to another.

Can't remove a post yourself if it's the first of a thread. There is an "Alert" function you can use to place an alert on the thread and an admin will be along sooner or later to remove it. You could also edit the mistaken post and change the title to something like; "Ooops, Please delete this thread." You can likewise alert a post that you inadvertently put in the wrong forum and an admin can move it to where you wish it had been.