> Anyone know of a service that provides drip pester-the-shit-outta-people messages until they pay you back?

There are federal and state laws about debt collection to protect debtors from aggressive debt collectors. If you harass someone for a debt that you can't prove they owe, you could get into deep trouble. If you continue to contact somebody after they tell you to stop, you can get into deep trouble.

> It can use constantly changing numbers and emails, so it can never be blocked.

This could be a violation of anti-spam laws.

> You can select how aggressive or sweet the messages can be.

Aggressive? If you threaten someone with violence, you're going to get a visit from the cops. If you threaten someone with reprisals like ruining their reputation on social media if they don't pay up, that's extortion. You could go to jail.