To cancel a OneCare subscription, you need to contact support:
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
Unofficial path - send me an email at sboots@mvps.org with a subject line including "OneCare" (to get past my spam filtering) and in the message body, tell me what country you are in, your forum nickname, and paste this link: http://social.microsoft.com/Forums/en/onecareinstallandactivate/thread/4f9795d7-7b8e-4d64-b1ae-9afa0841f0cb
I am not online 24x7, so you may be waiting for a reply, but I'll do my best to hook you up with support.
-steve
Microsoft MVP Windows Live / Windows Live OneCare Forum Moderator