At this point they are acting as collection agencies, and are subject to those rules.
You need to get orginized at this point. Get each bill they are trying to bill you for, get a copy of the proof of payment of that bill, and if you have a EOB for that service to show that you had paid your part, get that as well.
For each one, you need to write a letter stating that you have already paid for this co-payment, and that you do not owe it, citing the proof that you will send along with it.
Then tell them that if they continue to contact you stating that you owe this, you will consider that harassment and will take the necessary action to stop their harassment.
Send it registered mail to the medical office AND the whoever runs the hospital so that they are aware of what is going on, so that if it ever did reach court, you would have the proof that you sent this letter along with all of the proof to show that you paid the bill.
It sounds alot more scary then it is, but if you are SURE that you have paid for the service, then you need to stand up for yourself.