Thanks for the suggestions and comments, everybody; I think a combination of dropping the faucet reward again (I'll start bundling up faucet payments into 'sendmany' transactions, so the transaction fees are lower) and "community watch program" will work.
The 'community watch' will be a web page that anybody can see that shows the last 100 IP addresses that got payments along with an obfuscated version of the email address (I'll obfuscate by randomly turning an email address like 'gavinandresen@gmail.com' into 'gavniadresen23@gmail.com'). And I'll recruit some trusted people and give them access to a master faucet shut-off switch if it starts getting abused again.