Worst service ever

I went to cancel my contract in December at the shop as it was expiring the same month. The guy took my ID did something on the system & advised that it was cancelled. A month later a debit order went through my account I reversed because according to me I had cancelled it. Then another month another debit order reversed it again then call centre agent called to tell me that I still owe them and my contract was not cancelled she said contracts can only be cancelled by phone not by going to the shop which the guy at the shop did not inform me. It's been months I'm still struggling with their debit order which I refuse to pay because I cancelled my contract in time it's not my fault that their staff is ***********. They must back off my account because I don't owe them anything. What a *** company is this I'm never taking another contract with them.