Hello, @Roger Roger,
Welcome to the Microsoft Q&A platform!
Based on your description, I understand that you want to remove the newly installed add-in from both Windows and Mac devices.
For Outlook Client, you can manage add-ins in the Microsoft 365 Admin Center.
Please refer to https://learn.microsoft.com/en-us/microsoft-365/admin/manage/manage-addins-in-the-admin-center?view=o365-worldwide#delete-an-add-in for details.
For OWA, you can finish it more easily by following the steps and screenshots below.
1.Open Outlook and go to the Home tab. Click on the More options (three dots) button on the right side of the ribbon and Select Customize.
2.Under Buttons, check (add) or uncheck (remove) the command buttons you want and click Save to apply the changes.
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Thank you for your support and understanding.
Best Wishes,
Alex Zhang