Sending of letter of demand

I have a property in Ramsgate. I had a 36 month contract with ADT for a small alarm system. We decided to demolish the house due to all the crime and property having no value on the south coast. ADT was notified of our intentions and they removed their system prior to us demolishing the house. We also indicated that we will not renew the 36 month contract that only expired 20Jan 2014 ,some time after demolishing took place.<br> We were notified that we had to pay till end of contract and we did but they kept on charging even for Feb 2014.<br> Now Adt sends me a letter of demand for R4074.81!!!! Please ADT sort yourself out.