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.
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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.
How to force an app to close on your iPhone, iPad, or iPod touch.
When you log out of your Mac or restart it, deselect ”Reopen windows when logging back in” when prompted.
Or start up in safe mode, then restart normally. This doesn't change the setting, but your Mac forgets any windows that were open the last time you logged out or restarted.
If you deselect this setting and an app continues to open automatically:
To prevent apps from remembering the windows they had open, choose Apple menu > System Preferences, click General, then select ”Close windows when quitting an app.”
Or press and hold the Shift key while opening an app. This doesn't change the setting, but the app forgets any windows that were open the last time you quit the app.
If these steps don't work for an app, the app might have its own setting for reopening windows. Check the app's preferences or documentation, or contact the app's developer.