Click the app name in the menu bar
Select Quit
Press Command + Q
Right-click the app icon in the Dock
Select Quit
Press Command + Option + Escape
Select the app
Click Force Quit
Click the red close button on the app window
If the app stays open, use Force Quit from the Apple menu
Open Activity Monitor
Select the app
Click the X button
Choose Force Quit
Restart the Mac if the app will not close