Go to the Apple Menu in the upper left of the screen.
Choose System Preferences.
Click on Users & Groups.
Click on your Username.
Choose the Login Items tab in the middle near the top of the window.
Click to highlight the Discord App.
Press the minus button to remove Discord from the start up list.
Restart your computer for the changes to take effect.
Find the Discord App Icon in the dock and right click.
Choose quit.
Click on Finder in the dock (the Face icon).
Choose Applications.
Click and drag the Discord icon to the trash.
Right click on the trash icon to empty the trash.
Restart your computer for the changes to take effect.