MTN expect me to pay for ********** Activity on my contract

Over billed by @MTNza. From paying R450 pm for data for past 7mnths to 8.4K in Dec. After sent from pillar to post, I picked up from their feedback that ********** activity happened, limit was increased from R0 to 8K & purchase made on @MTNza app (not by me) never accessed app until recently. Now bill is 10K +, I'm told I must pay because I increased limit and made purchases. I'm so frastrated as I could never increase to even R600, let alone make ridiculous purchases.