You will need to disable all the notifications on client:
1. Go to Settings -> Agents/Plug-ins -> Software -> Patch Management -> Windows -> Default Software Update Plug-in Policy -> Notification tab"
2. Untick all the check boxes (or leave the desired notifications enabled/ticked)
3. Update configuration on the clients
NOTE: If you'll have reboot option set to "Never" under "Settings -> Agents/Plug-ins -> Software -> Patch Management -> Windows -> Default Software Update Plug-in Policy -> Installation Schedules tab" and all the check boxes disabled in the Notifications tab, then reboot will be performed/notified only when computer will be manually restarted. To avoid this set up a schedule for reboot.