Unauthorised debits

I have cancelled my car insurance with FNB, mostly because of the unsatisfactory service ive been getting from them. They have been debiting my account which is very irritating as i have to reverse their debit orders every month. I dont know how long will they keep on doing that but i certainly dont have time to be reversing debits back into my account. They hardly respond tomy queries but they never forget to debit my account even though i have cancelled with them.i dont know who should i speak to regarding this matter, but i hope they cancel billing my account. I do not need all this stress!!