The main reason why you may need to disable the system abort sequence (Stop+A) could be security of your system. Anybody who can abort the system and bring it to OBP can eventually try to gain access to the root user account. You can disable the system abort sequence (Stop+A) in several different ways.
The first option is a variable KEYBOARD_ABORT inside the /etc/default/kbd file. This variable is commented out by default and has disable value assigned.
Uncomment this variable as shown below
and run following command as root or reboot the system.
# kbd -i
This command will basically tell the system to reread the /etc/default/kbd file. You can enable abort sequence at any time by commenting out KEYBOARD_ABORT variable and running the kbd command.
The second option is the /etc/system file with a abort_enable variable. If you set this variable to 0 (zero) you disable all break signals on the system.
It is necessary to reboot the system after this change.
The third option does not require any file change. You can enable/disable abort sequence from shell using kbd command.
# kbd -a disable
By using disable option you disable the system from accepting the break signal.
# kbd -a enable
By using enable option you enable the system to start accepting the break signal.
Only first two options are permanent, the third one is valid till next reboot.