One option could be to charge a micro amount of BTC to join the site. Dont flame me for this suggestion
But perhaps a small fee of 0.00100000 ($0.22) would at least cut down on bots creating spam accounts.
I cant see 22 cents as being a reason why someone wouldnt join. A person with no BTC could easilly get that BTC amount from faucets in no time at all.
Just a thought!
Strato