more sophisticated system could be to use the real block chain and zero-confirmation payments through multisig.
like 2of2 with your game's master key and the player's private key, kept like a password seeded something.
then it would be like the player deposits certain amount of btc into his profile's btc address.
both signatures (his and the game's master key) are required to spend any money from thet address.
so once they make a transaction, it cannot be double spent again. not without both parties agreeing on it - solves loads of problems.
then, for the player's security, the profile address could be made in a way that after a certain period (e.g 6 months), he can spend these bitcoins with only his signature.
it's the new feature that nobody has really used yet, so suit yourself