As much as I'd like to help you, I can only direct you to the proper place to request cancellation of the service and a refund for the renewal charge - customer service.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
You have posted to a community support forum, meaning that we are fellow users of OneCare helping each other and discussing OneCare.
And, you did authorize the automatic renewal charge as that is part of the terms of the subscription that you agreed to when you initially purchased OneCare. I agree that canceling the service should be easier than contacting customer service, but they will take care of you and give you the refund that you desire.
-steve