Account verification: Only 1 account per person

Question

In my community, every user should only have one account.

So I need a solution to verify that the specific account is the only one the user owns. For the time being, I use email verification. But I don't really need the users' email adresses. I just try to prevent multiple accounts per person.

But this doesn't work, of course. People create temporary email addresses or they own several addresses, anyway. So they register using different email addresses and so they get more than one account - which is not allowed.

So I need a better solution than the (easy to circumvent) email verification. By the way, I do not want to use OpenID, Facebook Connect etc.

The requirements:

• verification method must be accessible for all users
• there should be no costs for the user (at least 1$)
• the verification has to be safe (safer than the email approach)
• the user should not be demanded to expose too much private details
• ...

Do you have ideas for good approaches? Thank you very much in advance!

Additional information:

My community is a browser game, namely a soccer manager game. The thing which makes multiple accounts attractive is that users can trade their players. So if you have two accounts, you can buy weak players for excessive prices which no "real" buyer would pay. So your "first account" gets huge amounts of money while the "second account" becomes poor. But you don't have to care: Just create another account to make the first one richer.