Press Command + Option + Esc
Select the app you want to quit
Click Force Quit
Click Force Quit again if prompted
Click the Apple menu
Select Force Quit
Select the app
Click Force Quit
Open Activity Monitor
Select the app
Click the X button
Click Force Quit
Right-click the app in the Dock
Hold Option
Click Force Quit
Use Terminal
Run killall AppName
Use Terminal
Run sudo pkill -9 AppName