When posting a reply to a thread, I got the attached message. I assume this means that a post somewhere in the thread (presumably already deleted) was spam?
Trackbacks and pingbacks are a system whereby websites can let each other know that their content has been linked to - often used to determine referrers and the like. I don't fully understand it, but I believe it only works when both sites are set up to send and receive then (vBulletin does by default).
I'm guessing that message is saying that the site in question has already been pinged for a trackback thingumajigger.