To force quit an app on a Mac, the most common and easiest way is to press the keyboard shortcut Command + Option (Alt) + Escape together. This opens a Force Quit window, where you can select the unresponsive app and click "Force Quit" to close it. Other methods include:

• Clicking the Apple menu in the top-left corner of the screen and selecting "Force Quit..."
• Right-clicking the app icon in the Dock while holding the Option key, then selecting "Force Quit"
• Using the Activity Monitor app (found in Applications > Utilities) to select and force quit the app
• Using the Terminal command killall [AppName] to force quit via command line

If none of these work, restarting your Mac or holding the power button to force shut down can be done as last resorts.