On the 10th of August, I returned the wifi to the Lakeside branch

On the 10th of August, I returned the wifi to the Lakeside branch . Even after they took it, they still deducted the R339. Just yesterday, i got a message stating that I am expected to pay R546 this month. I don't know how am I supposed to pay for something I don't have in my possession and not using. I need this contract to be canceled.