Would this work: $1 for ever (or because that might create an eternal obligation, $1 for 5 years) or some minimal amount just to verify they are real.

If it is a spam account they forfeit their $1.

This may create a hassle in that once in a while a false negative comes up, but you just return their $1 if they complain. (of course someone could defeat that system too but you should be okay).