Contract uncanceable months after term date HELP

I sent am email in April giving a months notice to cancel a data contract which expired in May... It is now September and I've recieved another bill. What has happened to MTN - I worked for MTN for about 20 years.i left there towards the end of 2019 - absolutely shocked... dont know what to do to get t hu is contract cancelled and a refund for all the months MTN has *******ly taken my money.HELp