Lawnstar does not honour their warrantee

I bought a Lawnstar edge trimmer from Builder Warehouse in April last year, in December the unit stopped working and I took it back to Builders Warehouse in Rivonia. After not hearing anything I started enquiring what was going on and eventually got a call from Builders saying I must pay R700 for the repair, no reason given just that I must pay. I asked them to get Lawnstar to phone me to explain why, nothing happened for a few weeks until I started asking questions again. Then I got a email from them saying I used the wrong cutting line and that's what caused the motor to burn out so I have to pay almost the price of a new machine to repair. When the cutting line for this machine was finished I had to go to their website to check what line to use, on the website it recommends to use 2mm cutting line. Which is what I bought, nowhere does it say that the warrantee is voided if you use the wrong cutting line. Now again I'm sending emails and not getting any response. The customer services is pathetic and Builders offers no help in resolving the issue. Buy Lawnstar products at your own risk.