Open OneCare on the hub PC. Click on Manage Your OneCare Circle. Then click on the PC name for the hub PC. The view will change to expand and provide the ability "stop being a hub" or some similar wording.
The other part is that I would expect that admin rights would be required to make these changes. So, a limited user should not be able to change settings, make a PC a hub, or stop a PC from being a hub. I would also expect that if a limited user was logged onto a PC that was made a hub previously, this might limit their ability to take actions when an alert appears about one of the monitored PCs.
-steve