Should you wish to remove the Appogee Leave add-in for Microsoft Outlook simply open the settings cog at the top right of your Outlook screen, navigate to Add-ins and look at My Add-ins.  When you locate Appogee Leave click the 3 dot menu to the right of Added to find the Remove option.  If you want to prevent new users from getting the add-in you will want to go to the Exchange Admin Center under Organization choose Add-ins and delete the Appogee Leave add-in from there.