If you want to comment out specific lines in a file you can use sed. Let's assume we have an "alloweduser"-file, and we want to disable all users starting with "adm" on a specific date.
this might be our /etc/allowedusers-file:
doctor rose martha clara adm01 adm02 adm03
now we want to comment out all lines starting with "adm" on 20th february 2015
dr@tardis:/etc/allowedusers# at 15-02-20 warning: commands will be executed using /bin/sh at> sed -i.bak -e "s/adm/\#&/g" /etc/allowedusers at>
job 5 at Fri Feb 20 14:24:00 2015
This will replace all lines with "adm" to "#adm.." AND it creates a backup-file /etc/allowedusers.bak with our old entries.