Or, an option at the top of the first page, like, "sell all IP adresses under my user ID to xx", (where, xx represents the user's country code).
Yes, there might be some users using many computers from a couple of countries under the same user ID, so one by one manual setting is also necessary. But I think at least 90% (probably more) of the users are contributing from a single country.
BTW, thanks for the option Louie. It's still much better than nothing.