Ограничение входа в систему

man login

If  the  user is not root, and if /etc/nologin exists, the contents of
       this file are printed to the screen,  and  the  login  is  terminated.
       This  is  typically  used  to  prevent logins when the system is being
       taken down.

crontab:
0 10 * * * rm -f /etc/nologin
0 18 * * * ln /etc/nologin.bak /etc/nologin