Refuse to exchange

I bought a cable for a tablet charger from Vodacom shop in three rivers in October 2015 , On the 14 November it was broken i could not charge with it and the tip that connects the cable to the adaptor was broken, i returned it they gladly exchanged it, now same thing happened i cant charge my phone the tip is also broken so i went there on sunday to exchange it again , i was told that they cannot exchange it because i broke it , they said i pulled it . My problem is i paid R189 for something could not last for 6 months . please help