DMC harassing me

They are harassing me for an account of more than 18 years with Blue Financial Services. The account was paid up in October 2010. Proof was sent to their offices after they sent me a letter of demand this year this is 13 years after the account was settled in full. They harass me daily after I said to them that they must get an court order because the account was settled in full but still they sent e mails and sms messages. They are not going to get any money from me and I am acting with the law on my side and even if I still owed money which I don't according to law the account had become prescribed because in 13 years nobody from this office or Blue Financial Services contacted me to request any money still outstanding.