I think charging $1 to register would be enough to put off spammers, and as you say, if they could use someone else's money to do it, then they'd just steal the money instead.
Or don't even charge a dollar, but distribute one-time auth codes through printed flyers, or internet cafes. Anything that requires you have some physicality. Once someone's signed up once with such an account, and not got banned for spamming, let them use it to register for other accounts, or generate a limited number of auth codes to give away.
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I think charging $1 to register would be enough to put off spammers, and as you say, if they could use someone else's money to do it, then they'd just steal the money instead.
Or don't even charge a dollar, but distribute one-time auth codes through printed flyers, or internet cafes. Anything that requires you have some physicality. Once someone's signed up once with such an account, and not got banned for spamming, let them use it to register for other accounts, or generate a limited number of auth codes to give away.