On Undernet, we've been being hit with a lot of female name + 2 digit number clones that end up spamming the users with porn messages. Thankfully, these are easy to ban with a simple script and I even offer a way for legitimate users to get on an exception list so they can still use a name + 2 digit number for a nick if they want to.
However, as RusselB mentioned, it is really hard to stop all of the different types of clones/bots because the nicks can have very normal formats and even legitimate users sometimes use nicks like dsfdsf (I think that's the nick of one of our regular visitors... or at least something similar).
