A response will simply encourage more contact. They really have only one weapon right now and that is to pester you. It would be a waste of money and resources to sue you, so I agree - ignore them until they sue and the likelihood of that is very minimal.
It just means that they're using a cheap method of trying to contact you. They've already spent money on two form letters, two envelopes and two stamps. They're trying to maximize their profit by getting you to pay them something.
You have a lawyer (or lawyers). Discuss it with them, but the odds are still with you - letters, then phone calls, then they let it go.