Every website that offers free faucet has to deal with this issue, it's normally wise to implement security measures before launching a faucet so you don't have the headache later. But there are a few simple tricks you can do that make it harder;
1. Prevent sign ups from temporarily email domains
2. Block all VPNS/Tor/Sock5
3. Restrict accounts to 1 per IP address, but don't make this rule public knowledge otherwise it will be circumvented.