Setting limit to homepage url?

[windrider07]

I've got spam bots registering like cray on my forum. I've installed a few plugins to help block spam bots from posting and have kept it down real good. Now my only problem is, the spam bots keep on registering and it looks bad on my members list. They aren't spamming my forum anymore but they come, register, edit their profile page so that their homepage url contains a link, and then leave. How do I set it so that a new user cannot set their homepage url unless they have a certain number of posts? Trying to prevent spam bots from registering or doing anything on the forum.