You want to prioritize signatures deployed by the Outlook Add-in by removing the built-in signatures options from Outlook on the Web (OWA).
You can remove the built-in signatures options from OWA by disabling the feature in the OWA mailbox policy in Exchange Online.
Please click on the required options listed below to go through the detailed description:
Navigate to https://admin.exchange.microsoft.com/#/owapolicies.
- Select the OWA policy. The details of the selected policy are displayed on the right-hand side.
Under Features, click Manage Features.
Expand the User experience drop-down, and untick Email signature.
- Click Save changes.
- Open a PowerShell window and connect to Exchange Online.
- Run the following command to disable Outlook Web signatures from all mailboxes: