I'd set up an on-going correspondence with their Solicitors.
Solicitors charge for every letter sent *and * every letter received. If you reply to their Solicitors, the Solicitors will write to this couple explaining (and possibly enclosing a photocopy of your letter; for which they will also charge) and asking for further instruction.
As you are not doing anything wrong, except living your life, content yourself with getting a small revenge by hitting them in their wallets.
You'll just have to learn to ignore their behaviour, I'm afraid.