1. Press the Windows key to bring up the dash, type "gnome-system-monitor" in the search area, then open System Monitor.
2. In the System Monitor window, open the Processes tab, select the program you want to force close its process, then click End Process.
3. Another window will appear, confirm by clicking again End Process.
4. Or, you can right-click on the program you want to kill its process and select End Process / kill Process.