There might not be a need for one at this point, but if you think it's worth keeping it around, the following might be relevant.

Instead of having an option for each coin, have one for a site-only currency, let's call it Gary.

This Gary would be pegged to one of the other cryptos in terms of value.

Only staff members would be enabled to send it to other users, while regular users would only be able to exchange it after having reached a minimum to be set.

Other bonuses given out by the site could also be in this new currency. This in turn would help prevent people from sending these bonus funds around through their various accounts, and stacking them up.

Now, what might be interesting is to have the faucet function as a lossback of sorts, this by having it scale up depending on how much one's lost on that account.