fire up a console / terminal / TTY / whatever commandline tool you prefer to use, and do this:
There are two ways to kill a process (for eg. an app), kill by pid (proccess id) or via killall and the processes name..
ps aux # look for the apps name and find its pid
kill -9 pid
killall -9 app_name # the process name isnt allways the same as the app name, ps aux can help you find out the process name also..
Upon killing the process, the application dies.. the signal 9 doesnt leave any other choise for it, a tad like the windows shutting down of processes, but more powerful.. If you wana let the process a chance to close more peacefully, use the code 15 instead of 9