I'm very newly endowed with posting privileges, so this is a completely legit question... Where is this fabled "dismiss" button?

I admit I haven't really looked for it, except in cases where I have already engaged someone in discussion, only to later realize they were a troll, or an asshole (is there a difference, really?)

Given this situation, and the fact that at least 2 other contributors were given posting privileges the same day as me, a tutorial on the use of the dismiss button would be a welcome thing.