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.
Click on the background graphic or otherwise make sure you’re in the Finder: if the system is trying to switch you to the stuck app, it probably won’t be able to pull up the menu necessary. Easy way to tell: Look on the top left and see if it says “Finder” adjacent to the Apple menu. Click on the Apple Menu and look for “Force Quit”. Option #1: Press Command-Option-Esc keys to force quit full screen app. To open “Force Quit Applications” on a Mac OS X press Command-Option-Escape: Fig.01: Force Quit App. Select an app (say VLC) and press “Force Quit” button. You may be prompted as follows: Fig.02: Confirmation dialog box.
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How to force an app to quit
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.)
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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
How to force an app to close on your iPhone, iPad, or iPod touch.