Closing the account would force them to contact you in writing before they can just pull the money out. I wouldn't spend any of it until at least two years, maybe even three, have passed. Accounting errors usually get caught during the annual cycle, but may take two to become apparent. Once somebody finally realizes it (even though you told them about it) they'll finally take steps to fix it.

You would think a bank would be better about reconciling accounts. Then again, if the correct account they should have credited belonged to some yuge corporation that was not expecting the refund, they will never miss it and never complain about it, giving the bank no reason to believe there's an error. Like I said, give it a couple of years. If it's not corrected, go to Harrah's and bet it all on black.