If you really want to be rid of these, call US, tell them "Place my number XXX-XXX-XXXX on your "do-not-call" list. If you get one more call, notify your state AG and the federal trade commission. Even with a "pre-existing business relationship" if they continue to call, they can be subject to about a $10k fine. (this assumes that your phone number is already on the FTC do-not-call list)

I would only do this if it continues to happen after you call in and try to have them resolve it. If they cannot, I would not hesitate to do this--it might be the only way to get US to fix the problem.