Never thought of this until now but what if you had some kind of pay system that worked more like an escrow-ish service ?

So initially you have to pay some small sum. Eventually sometime not too long after (Hours? Days?) your money is returned to you. Make it pricey enough to deter large bot spam (some will of course still pay for single bots) and cheap enough to be affordable for most.

Like putting a quarter in a shopping cart !