To quit (close) a Mac app normally, choose Quit from the app's menu in the menu bar, or press Command (⌘)-Q. If the app doesn't quit, follow these steps to force the app to quit.
How to force an app to quit
Sound Control enables application-specific volume sliders on the Mac. In addition to Menu Bar controls, Sound Control can also be configured with user-defined keyboard shortcuts to adjust the level of a foreground app, or mute any background app. To access this super quiet mode, use the volume keys on your keyboard to manually set the volume to zero—this trick won’t work if you use the slider in the menu bar’s volume controller.
Press these three keys together: Option, Command, and Esc (Escape). Or choose Force Quit from the Apple menu in the upper-left corner of your screen. (This is similar to pressing Control-Alt-Delete on a PC.)
Then select the app in the Force Quit window and click Force Quit.
You can even force the Finder to quit, if it stops responding: Select Finder in the Force Quit window, then click Relaunch.
Learn more
Mac Quiet App Volume App
How to force an app to close on your iPhone, iPad, or iPod touch.