This article will talk you through the steps of how to disable both Outlook's built-in report button and Microsoft's Report Message add-in.
How to Remove Outlook's Built-in Report Button
Outlook's built-in report button:
To remove the built-in report button navigated within Outlook, go to the Microsoft Security Admin Center: System> Settings > Email & Collaboration > User Reported Settings (https://security.microsoft.com/securitysettings/userSubmission) and select 'Use a non-Microsoft add-in button.'
How to Remove Microsoft's Report Message Add-in
Microsoft's Report Message add-in button:
To remove the built-in report button navigated within Microsoft, go to: Microsoft 365 Admin Center > Settings > Integrated Apps. Locate the 'Report Message' add-in. Open the add-in and click 'Remove app'.
Please note: The changes made above can take up to 24 hours to deploy and show results.