How do I forcibly shut down a specific application at a pre-determined time?
You can quit apps with AppleScript, such as:

osascript -e "tell application 'Safari' to quit"


Therefore, you can add the following to your crontab (env EDITOR=nano crontab -e) to quit an app without saving at 17:30 every day:

30 17 * * * /usr/bin/osascript -e "tell application 'Safari' to quit saving no"


If you want to quit without saving, you can remove the saving no.
Alternatively, you can kill the process with /usr/bin/killall Safari.