The best way to stop the faucet abusers is to give the faucet only when the balance is NIL. In this way, a person cannot claim another faucet unless he has lost all the faucet amount in gambling and of course there is no way to withdraw the tiny faucet amount.
Not as simple as that, in fact most faucet on gambling sites are able to be claimed once user's balance is Zero but still there are many people who tried to abuse it.
Faucet abusers will keep on claiming the faucet only with no intentions to do real gambling by making deposit. Once they claim the faucet, they will use it to play, if lose then they will come back again.
They can create many accounts for faucet especially if the amount of the faucet is good enough. That's what we call with faucet abusers.
Don't let faucet abusers to withdraw, problem solved. Only allow to withdraw if the player deposited and wagered the same coins. The other way to fight abusers is, making the claim amount so little so it will be impossible to make big money. (like 1 satoshi) I don't see any other way.