debited for service already cancelled in the previous month

I had used rain for over a year, and honestly, it was the best internet service provider in the beginning. customer support, easy use of the application, smooth internet etc. though, some time in February and March the internet started giving problems, and I wasn't even able to do simple things like send a whatsapp message. in March, I cancelled my SIM card by mistake, with the intention of leaving it dormant until another time. cancelling it wasn't too bad of a thing, and with confirmation that the sim card was cancelled on the 31st of March, and no longer active, I decided to leave it at that. on the app, it stated that it wanted to debit me in May, I queried it, and was informed to ignore those messages as it is a system error. come the 25th of April, boom-- DEBITED. a whole 480 rands debited for a card that was confirmed to be cancelled on the last day of March, I queried the situation, and person with the surname Tuli, spout some bull**** nonsense to me about how the card was still active. I'm pretty disappointed in this company, I have all the proof that my account was settled in full, and cancelled, yet this person still had the audacity.