they are still trying
They got a zillion of forum url's, and using a zillion of ever changing ip's to do the work, and they will try blindly to register and post, not caring about success or failure. That's the factual situation, and it will not change.
So, only way is to find the best way to cope (small, anonymous forum here, but am still getting approx 5000 registration attempts\week). Banning some
particularly aggressive ip's or ip ranges (in .htaccess), different mods, verification questions (the questions works perfect here, a few forum related questions and changing them now and then).
About .htacces, had complete country ban (ru cn ua etc), worked fine for a while, but in the end the immense .htacces started choking the server, issuing random 403's + slowed everything down.