The Software Update Cycle ran and now computers are showing the reboot notifications configured in the Default Software Update Plug-in policy. Reconfiguring reboot settings in this policy has no effect on the pending reboot notifications.
The only way to stop a pending reboot is to stop the Symantec Management Agent service (AeXNSClient). This can be done by sending a run script task to the clients that runs the following command: net stop AeXNSClient. Unfortunately the reboot will proceed when the Symantec Management Agent service is started again. Also the service can not be started via task because it needs to be running to receive a task. Use a group policy object (GPO) to restart the agent service on a large number of computers.