I'm not sure, but last summer when they had that promotion of two months of Xbox Live for $2, I took it. The thing is that it required that I had a credit card placed on my account, so I placed mine there. Before the two months was up, I wanted to cancel my auto-renewal but I found out that I couldn't do it manually. When I went to the Xbox support forums, the common answer I saw there was that the only way to do so was to call customer service and ask for a cancellation. I did so, and they removed the auto-renewal.
tl;dr: If everything is still the same as the past summer and as far as I know, the only way to cancel the auto-renewal is to call customer service and ask to remove it.