You are using Microsoft 365 and wish to disable the built-in signature functionality in New Outlook and OWA.
The Exchange admin centre provides the functionality to disable this feature using the Outlook web app policies feature within the Roles menu option.
The steps below detail how to disable this option for all members of your Microsoft 365 tenant. If you wish to disable this for specific users, you will need to create a new OWA policy and apply these settings.
Log on to the Exchange admin center and select the Outlook web app policies option from the menu
Click on the policy name, and select the Manage features option from the fly-out
Expand the User experience drop down, and deselect the Email Signature option
Your users will no longer see the Email signature option under Compose and reply in the Outlook settings window