Did you ever make any headway on this one? I have seen some systems make the email field unique in the member tables which solves this problem but it was probably not made unique in DNN for a very good reason. There are probably situations where you want to be able to allow some users the ability to use the same email address with several different accounts.

I think what you are looking for here is the ability to have a lock table which prevents certain types of people from registering on your system which I am guessing is for anti-spam purposes. It would be useful to be able to click on an existing user and say: "add last IP address and email address to ban list" that way, no one using that IP or email address can register on the system again. This would also stop the attacker from using a different email address from the same IP until they have refreshed their IP address. Of course, if they're using a series of anonymizers then that won't work.