Mellerware two plate

Yesterday I went to spar in Peddie to check on my appliance "the mellerware two plate" which I did return to them. Due to the left button which was melted. And the nob wasn't working properly due to burnt. The stove is still have guarantee and what surprised me from them is they are not going to replace or refund me. They said it's my electricity fault. Please advise what to do next. Thanks