I thought the slogan was pay for the "data you use" in which case I'd expect something like no limits on how many in/out per day but e.g. first 1GB in/out is free and $0.10/GB after that with no limits as long as you pay. Or something like that.
As for managing spammers, just require some sort of ID verification? Or set the price point high enough that spammers wouldn't be interested?
$1.50/month for personal or $16/month for business is insanely cheap, I pay a lot more than that to Google. If you're too inexpensive, spammers will be attracted to your pricing.
Real businesses would have no problem paying upwards of $100/month, to be honest.