We have received notices for two au pairs for unpaid medical bills (charges were outrageous and deceptive, and they still did their best to resolve with insurance, but in the end left with unpaid bills).
We did not receive calls but we still get mail. They will peter out after a while, but it's going to take a while. I agree with blocking the phone numbers, and either put mail in the garbage, send back to sender (unknown recipient) or send it back to her parents house (you should have the address from the original placement?).
You ex-AP sounds like a piece of work...