How many public Tor exit nodes are there these days? You've essentially blacklisted 10 of them already
I am going to add a one-donation-per-bitcoin address rule to make it a teeny-tiny bit harder to cheat. I don't want to blacklist anybody coming from Tor, though-- I think a lot of people using Tor will also be interested in Bitcoin.
I'll implement a CAPTCHA when (if?) the current service starts running low on coins (there are 1,570 bitcoins in the faucet right now!).