There's no need to cancel, since all sales of OneCare have already ended and only free six month extensions were being given out, which I belive have now finally also ended.
Windows Live OneCare End of Life Guidance - Updated
http://social.microsoft.com/Forums/en-US/onecaregeneral/thread/81996436-edad-41ba-bdfc-02fc094e2ca9In any case, you will not be charged for any renewal, so there's really no reason to waste your time trying to cancel.
Rob