NSEmailMax will fix that, set it to a low number such as 1-3 this makes it so only N number of nicks can be registered to the same email address, there is also an older anope module called os_blacklistemail that will allow you to blacklist entire domains from registering nicks. In addition to that there is also another module called ns_dropbyemail that allows you to do exactly what the sane says .. drop by email address.
[Edited on 2-8-2007 by katsklaw]