Press Command + Option + Esc
Select the app you want to quit
Click Force Quit
If the app is frozen, click Force Quit again if prompted
Click the Apple menu
Choose Force Quit
Select the app you want to close
Click Force Quit
Open Activity Monitor
Select the app
Click the X button
Click Force Quit
Press Command + Option + Shift + Esc for a few seconds to force quit the frontmost app on some Macs