Use slower / heavier hash -- PBKDF2 or better. For lulz use something like Darkcoin Cryptocurrency 'X11' Proof of Work function - multiple rounds of 11 different hashes. Or more. (X13, X15 and X17 all exist as PoW functions in CryptoCurrencies today).

A <$1000 bitcoin (SHA-256) mining ASIC appliance is likely to be doing 1TH/s. Makes 2^16 rounds look kinda weak.