You could always do something similar to
this.
The next letter I got would be returned with a covering letter saying that if they continue to contact you incorrectly then you will have no choice but to charge them for your waisted time. I would send the letter recorded delivery and maybe even mention something along the lines of them having to opt out of your proposed charges within 14 days of receipt, and point out if they don't specifically tell you they're not going to pay you, then you can assume they are agreeing to the costs involved if they send anymore letters. I would set out my (reasonable) costs too so they can't say you didn't inform them.
Then if the letters continue send an invoice, followed at a suitable interval by a reminder, then it's a trip to the local County Court. It worked for the guy in the link so it could work for you. Just think you could end up emplying your own debt collectors.....
Cheers
Grim