Can you provide more details? Are you trying to remove the hub PC from the Circle on the hub PC or a different PC? Do you have more than one PC set as a hub PC? How many PCs are in the Circle?
If you are trying to remove the PC that is the hub, first change it to not be a hub PC and save.
Then, go to another PC and make that PC a hub, and select to remove the old PC.
Note that removing the PC via the Circle status on a hub is optional and will cause OneCare to become unsubscribed on the target PC. You can also simply uninstall OneCare from the target machine with the same results.
If you are unable to complete this, you can contact support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator