I would probably return the money, I don't need to steal. But having your balance stole? No, this is a unacceptable way of doing this, and that's not the balance, they even let me play it and so took my winnings as well.
Then what are you complaining about? You wanted to return the money, so you deposited to return the money. You have no case. They couldn't steal anything because:
1. It's their money
2. You just said that you wanted to return it
I am complaining because they let me play my new deposit, make new deposits and entertaining with a money that weren't "mine" as you said, If I was advised that my balance were negative I could make those bets with my money and would have won like 0,2 BTC's.