It is very frustrating to hear you were mistakenly told to call to cancel. I believe that is the root of the problem here and I sincerely regret that you were told this.
To cancel a plan within 30 days, it needs to be completed in store. It is actually processed as a "return" of the service plan just like any other product. Unfortunately, when you called to cancel in December, the request could not be completed because it hadn't had it's first recurring billing yet. The agent you spoke to should have known this.
I do see that the plan is actually cancelled now, per your call on the 7th. I've sent you a private message with additional details. To read it, please make sure you are logged in and then click the envelope icon in the upper right of this page.