You may find that your existing users have a signature configured in their 'Mail Preferences' for their e-mail.

It is possible to disable these 'local' signatures for a specific group of users using an esig console command.

First create a group in your Domino directory containing the users that you want to disable local signatures for.

From the console load the 'esig task'
>load esig
and enter the command
>tell esig disableuser 'groupname'

Once the task has finished you can quit the esig task.
>tell esig quit

NOTE :- The task updates the users Calendar profile document. By design, 'Profile Documents' within Notes are cached once opened. The only way to refresh a cached profile document is to re-start Lotus Notes.

You should try and run this command when most users will have their Notes client shutdown e.g. out of office hours.