Preventing out-of-state customers from using invoice in webstore

Thought I had an easy solution to this problem, but now I'm not so sure.

We're implementing a policy that says that all out-of-state customers must pay with credit cards at checkout (and only in-state folks get the option to Invoice).

I'd mistakenly assumed that setting their credit holds would solve this problem; though now I realize people with credit holds can still submit orders with invoices on the store.

Has anyone else gone through and done something like this, and how did you get there?

Thanks!