Multiple Users with same email?

So i have been building sites for a long time. I use a wide range of systems and build my own CMS's and custom solutions. i like vshare a lot, good coding and good features....BUT....

i have been having trouble with a user at my site (linuxtubes.com). the user is signing up ten times a day from different IPs (using proxy servers). The user is using the same email address on every signup, is there really no preventive measures to prevent the same email from being used on multiple accounts? I think vshare is great but know i think the team needs to focus on security and such, a ban email and user name feature is highly needed here.

Let me know what you guys think.....