Some image verification systems have been cracked sufficiently well to allow automated signups. In addition, it's not entirely impossible that low-cost humans could do this, too. How many signups are you getting?
I have been deleting about 4 or 5 of these a day for months now, blocking the use of .ru based email addresses slowed it a bit, now I am seeing mostly gmail addresses on them. At first, I only deleted them if the posted a junk website in their profile, then I learned about sleepers... Now, I delete any new signup that does not post on my board within 24 hours or ASAP if they include a junk website in their profile.
May I suggest your over-zealousness to rid your site of spam sleepers is hurting your ability to have users come back later?
Removing everyone who does not post in 24 hours is going to seriously hurt your ability to grow your forums in the long term. I may register with a forum one day and look around and then come back 2 months later and want to post.
While it is a bother to have these spammers and to chase them down... it is not worth losing users over, so I think you might want to re-evaluate the effect of your strategy on people using your site.
Do you always post in the first 24 hours to a forum you join? I have probably registered on 1000+ forums in my life, and I imagine the number I posted on in the first 24 hours was well under 5% of those.
The second time I came back and my account did not work, I would think it odd and re-register. The third time I would think your forums broken and never come back again.