Press Command + Option + Esc
Select the unresponsive app
Click Force Quit
Click Force Quit again to confirm
Control-click the app icon in the Dock
Press and hold Option
Click Force Quit
Open Activity Monitor
Select the unresponsive app or process
Click the X button
Click Force Quit
Press Command + Option + Shift + Esc to force quit the frontmost app
Press and hold the power button to shut down the Mac if it is completely frozen