how to kill a program in linux code example

Example 1: how to force quit application in ubuntu

One way is to hit alt+f2, then type xkill and hit enter. 
Now click on the program you would like to force quit.

Example 2: how to kill a process with linux

ps ux #list the running proccess
kill -9 <PID>

Example 3: how to kill process in linux

kill <processID>