Force Quit Process by name

You can use the shell command killall to kill a process by name. To incorporate that in your AppleScript you'd do something like:

<pre class=command>tell application "System Events"
-- is it running?
if (application process "Safari" exists) then
-- kill it
do shell script "killall Safari"
end if
end tell</pre>

Note that you can also eschew checking if the app is running - if it's not, killall will just throw an error which you can trap via:

<pre class=command>try
do shell script "killall Safari"
end try</pre>

If it's running, it will be killed. If it's not, the try/end try will suppress the error message.