[BirdOPrey5]

I don't know that there's really a way to stop this... If you use one of the existing "AE Detector" mods you can flag when someone makes a new account when they already have an existing account, but to block referrers by IP could cause unintended issues- for example if you have a member and they refer a family member or co-worker it's likely they will all have the same IP Address at registration even though they are legitimately different people. 200 people in my office would all show the same IP and there were at least 4 of us members of one particular site at one time.