To access and disable the Outlook add-in, use one of these two methods.
Use Outlook Desktop, click File... Manage Add-Ins. A browser will open OWA, and show a dialog with All Add-ins. Click 'My Add-Ins' and use the "..." menu to remove the custom add-in.
Navigate to https://outlook.office.com/owa/?path=/options/manageapps in a new browser tab. The browser will open OWA, and show a dialog with All Add-ins. Click 'My Add-Ins' and use the "..." menu to remove the custom add-in.