FastSlots what kind of deposit system are you using? Also I would recommend adding a check for 1 confirmation before crediting balance, its currently instant and what I could have done was yolo bet on 4x and if I lose, just double spend it back before confirmed, and if I win, well let it confirm and withdraw winnings.
We are currently using the helloblock api, but we are switching to chain.com now. I'm really very happy with chain.com, I think its the best blockchain api out there.
I'm aware of the double spending issue. We are monitoring to wallet closely and will react when someone does what you describe. Probably we'll require 1 confirmation on deposits in the future.
Best solution in my opinion is to check your log of deposits to that user (if you have one), and if ANY deposit is unconfirmed for that account, disable withdraws. This would allow for instant deposits, to make it instant to play here, but also safer for you.
Chain.com is very good and I use them, they have super fast response times and the blockchain api had been banning me due to too many calls.