To cancel your OneCare subscription you must contact support, typically by phone. However, the process outlined here:
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
does not make it easy to reach support. You can open a support case, indicating you are a subscriber, by using the procedure for a trial user via email.
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-US/onecareinstallandactivate/thread/96acda1a-a259-4024-a6cd-e26d7ebebf7b
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