If you are using Crossware Mail Signature to append server signatures at the server level, you will probably want to disable the users current signatures contained within their Mail Files. There are a number of ways that this can be accomplished.
Options to disable signatures being appended at the client
Modify the Mail Template
This option requires you to modify the users mail files so the functionality of adding the signature is disabled.
This require someone with designer skills to make modifications to your mail template and apply the changes to all your users' mail files.
When an upgrade to domino occurs, these changes may be lost or need to re-applied to your templates.
Use the Crossware Mail Signature 'esig' task
The esig task contains a command to update the users calendar profile to de-select the Insert Signature check box option. See Disabling users existing email signatures for detailed instruction on how to achieve this.
Use the Administration Process and Policy
If you are using an 8.5 server and mail policies - you can have the Administration Process update the user calendar profiles.
Disabling users signatures using a policy