Charges for what?

So I buy my own data but I am charged cause of the amount I didn't use. From the first email they can't send me proof of what I'm being charged for. When I used that extra amount. Months back when I used their money they made me pay for what I used now I have to pay for something I didn't use?

I buy airtime with my FNB app. The minute it's in my account they take some of the airtime.

Now they block me.. for what? Money they can't explain why they are charging that. You keep on saying limit. I'm ****en ****ed off rn