Still Charged for canceled lines

End of January this year I notified Afrihost of canceling a line and opening another later (March this year).

All good and well except I am still being charged for both lines. I call and they say oh OOPS SORRY YOU SHOULD NOT HAVE BEEN CHARGED and after a whole rigmarole I get a credit for the next month. Next month comes along and I get charged a random amount, this month it was for a pro-rata speed upgrade...

Whenever I see another (double) charge I phone and they CONFIRM THAT THIS SHOULD NOT BE HAPPENING, THEY 'CAN SEE THE LINE IS CANCELED' yet they say they 'don't understand it so cant fix it' and say I will be contacted by a manager. OF COURSE THE MANAGER NEVER CALLS. and I am charged again the following month.

How is this possible?