How do I cancel a.99 a month charge that should have never been put on my account by one of the Apple techs at a Apple store?

A tech at the Altamonte Mall in Florida signed me up for extra Cloud storage that I didn't need and now I get charged $.99 a month for the last couple years. I got 5 gb free when I bought the computer which I can't live long enough to ever use. How do I stop the charge for something that should never have been added to my account?