permission for a command
 

how to grant/reject permission of a command to a normal user(e.g. reboot ,shutdown)

I guess there are numerous ways you can achieve this...like, if some group has access to the command(s) or file(s), then just add/remove the user to/from the group. or set the permissions so that only certain user has rights to it, like root, if you need that kind of thing...you can also create a new group that has access to the command and then add all the users you want to that group etc.

Some distros have a group for shutdown and you need to remove them from that group. Your distro may provide a GUI app for doing so.

For shutdown you could just
chmod 750 /sbin/shutdown