Harassing me to pay money, that I don't owe them

Dear Sir/Madam<br> <br> I was told by DYNAMIX HEALTH& FITNESS account ECTH1395 that If I need to cancel the contract I must bring a person who will take over my contract. Which I did, and the latter I received call that was in 2014 then I expalined. They remained quiet until now 2015 April. got sms from Collections Manager: Credit Worx *** or ***. With Ref ECTH1395 with Balance of R1500.<br> <br> Can you please stop this people from harassing me, I don't owe them I took 2 year contract and used it for a week after that I come with replacement for my contract. this is just a day light ******* from this people