You already are using image verification to limit the bots. So the real culprits are probably real people.
Don't know if there is a perfect way around this but anyway.
1) Log on as a admin Goto mystuff>Edit Preferences.
Scroll to the bottom. There is a option to notify you of new registrations.
2) I don't think so. At least I am not aware.
3) The only way I know of is possibly set up a newbie group. And allow that group to post in a specific forum. If you object to what they posted. Then you have the option to ban them.
But to be honest it would be the same as allowing a guest to post.
Then if you ban them for whatever method(IP,Email) they will just come back as another email or ip.
