Currently the process the user goes through is register, then verify, then gain access to the site. There is no build in functionality for blocking specific users, but there are a couple of things you could do with the Solution that could give you this type of control.
One thing that you could do is to remove the verify email code so that the user does not get the verification link. Then only users that you manually verify will be allowed.
Another way you could do this would be to have a table of banned email addresses and copy/modify the code on the registration page that checks if the user has already registered. If you copy then update this code to do the same thing but look at your banned users table it could stop those users from registering.
Both of these techniques are not supported within the User Registration Solution Pack, but they are examples of how something like this can be done.