Is there a way to stop Bad Bots?

Hi there,

I have bots adding a multitude of users to my multisites network. I have slowed them down, but I cannot find an effective way of stopping this. They seem to able to bypass the signup process altogether. This is a new network with really nothing to see, so I don't understand why they want to do this..

1. What is the reason these spammers add users to blog networks?
2. Is there any potential harm from it?
3. After I enable blog creation, will they be able to create new blogs the same way?
4. Is there a way to require email confirmation before the user is created?

Thanks,
MJ