The next thing that will be done--before markets are unfrozen--is a daemon will be created that continually monitors for negative balances and freezes any account with a negative balance
This isn't the right way to fix the problem.
What you need to do is to make sure that users aren't allowed to do two balance-affecting things at the same time. Otherwise they'll just find another way to cheat you.
Make "check balance" and "reduce balance" atomic.
Checking for negative balances isn't the answer. Suppose I have 30 BTC and try to very quickly withdraw 10 BTC twice. Both "check balance" calls see I have 30 BTC, which is enough. Both "reduce balance" calls set my balance to 20 BTC. Then you send me two separate 10 BTC payments, my balance has never been negative, and I'm 10 BTC up on the deal. You need to make sure that the "check balance" and "update balance" happen without anything else relating to that user happen between them.
This is a security issue that has been documented for weeks. OP just didn't keep up with security patches.