Sometimes some applications freezes up when we try to run it and we cannot close it. Instead of having to restart your system and wasting valuable time, we can simply use xkill to terminate the application so that you can continue with your computing tasks.
1. Press Alt+F2 on the keyboard.
2. When the Run Application window appears, input xkill in the textbox and click the Run button.
3. The cursor will turn into a skull and crossbones. Move the cursor to the offending window and left click your mouse to terminate the application.