Still being debited after contract ends

I received a phone call from Virgin mobile in June explaining that my contract was expiring at the end of July and would I like yo renew it. I said no for numerous reasons and I would like to cancel the contract at the end of July. The caller said no problem and hung up. Come the first of September and I'm still being billed. And on top of it my usual R409 debit has risen to R509! This is unacceptable and after getting exactly zero help from the helpline I'm going to put a stop on the debit order. Unless virgin can rectify this situation by some miracle.