The email address is not a good thing to depend on to check for cheating, I'd be checking the IP (as others mentioned) and stats on how many entries share the winner's postal code or same street (and check the address is valid) and take the phone number too if posible.
One of my ISP's mail server (IMAIL) features allows you to include a mail folder name in the email address (normally using a dash)
eg
myaccount-mailbox@mailserver
Set the account to forward all messages and they even end up in the same folder, so much like spamgoumet you end up with almost unlimited, instantly creatable "unique" email address.
So he'd need to block email addresses with hyphens in it (or to be really effective any other punctuation characters too
One think I won't do is enter a competition which rejects an anti-spam address as I'd assume they were intent on spaming.