Cancel expired contract

I've been trying to get hold of mtn for the past 3 past weeks. I'm trying to cancel my expired data contract but they aren't picking up their phones. I went to mtn shop in Alberton Newmarket but I got told that they no longer cancel contracts at the shops but i must keep phoning the 135 number. Luckily today I got through to the rude consultant and she tried to sell me other contracts which I wasn't interested to them but when I told her no I'm not interested to any contract she ended the call