I notice new posters periodically appear in the Off Topic forum out of nowhere and diligently post replies to every single new topic without fail, no matter how wide-ranging and disconnected the subject matter might be. The user amandasmith was one such poster, posting an impressive amount of replies in a short time, but now I can't find any trace of this user.
I deleted her completely. Started by a PM to her and even got a meaningful reply. However, later after few more posts I had seen, I was sure that it's only about spamming. Hard to say whether these users are fully automatic spambots or to some degree controlled by live users. Anyway, they are painful and I'd recommend all moderators to be merciless to all new users that post only a lot of posts, without any real meaning.
I presume they might be smart spammers. First enter a forum and post semi-meaningful, but not really contributing messages and later change the signature to include spam when nobody is paying attention to the posts anymore.
A user (and all his posts) can be deleted only by Admins, it can't be assigned as a Moderator right. That said, it looks like it would be useful to assign this right to Moderators (even if a little dangerous).