As has been pointed out in recent posts, we have an automated anti-spammer which occasionally ends up with false positives (ie, you're detected as a spammer when you're not). We go through these lists occasionally and "unspam" bona fide posters.
However, we've not been able to date to prevent human spammers (as opposed to robots) from creating id's on the site, and as a result we're drowning in spam users (which I suppose is a good thing in a way, if the site wasn't popular, they wouldn't bother trying to spam it).
So, we've decided from now on that such users will be systematically blocked, then deleted, from the system. And we've also taken steps to tidy up the dud users already on the system. Unfortunately, it's not impossible that a real user gets deleted by mistake. If this happens, then we apologise and suggest that you ask us to reinstate you using the contact form.