In the McAfee Agent policy you have 2 options.
1) show pop up
2) force reboot after certain time. You can use this 2nd option to force the reboot after a while
As far I know, 'Cancel' button can't be grayed out. Please submit PER if looking for such feature.
If you select the second option ''force reboot after certain time" then client machine will be forced to reboot without user's knowledge and user's may loose unsaved data.
The best way will suggest to convince/request user to reboot the machine.