MainStage also includes a Panic function, which works like the Panic function in Logic Pro. The Panic function immediately silences any hanging MIDI notes.
Press Control-P.
Double-click the MIDI Monitor in the toolbar.
If the Panic button is visible in the toolbar, click it.
If you have mapped the Panic function to a screen control, press or move the corresponding controller.
If the Panic button is not visible in the toolbar, you can customize the toolbar by adding a Panic button. For information about customizing the toolbar, see Customizing the Toolbar.