This is an interesting trick that I noticed if you are running KWin: you can very easily forcefully quit a program. Press Ctrl + Alt + Esc, and the cursor is replaced by a skull. Then click the program you want to kill, and it is gone immediately. If you accidentally pressed the shortcut and you don’t want to kill any program, just click on the right mouse button to dismiss the function.
If this doesn’t work, the keyboard shortcut may not be set. In that case, go to the Global Keyboard Shortcuts, pick component KWin and set Force Quit Window to Ctrl + Alt + Esc (or any other shortcut).