I must ask, how are you planning to enforce such a rule?
Smart people can just mask their IP (I won't explain how), and you never know if a user is using a dynamic or static IP, anyway. Therefore, there is no way to enforce such a rule.
The only thing you can do is watch for when the accounts request prizes, but what if those accounts ask for digital prices? Even Steam users can have multiple accounts to redeem games on, and Steam key codes do not even have to be redeemed by the winner. They could give those codes to friends or even sell them if they are savvy enough. o_O
I don't mean any offense, just trying to point out a flaw in the logic here.