Can the Paubox footer be removed from email messages?

No - the Paubox security footer is not configurable.

The security footer ("Secured by Paubox - HITRUST CSF Certified") is designed to let your recipients know that your email was sent securely with encryption. It's a global stamp that our outbound servers apply to every email that goes out via Paubox, and we don't have the ability to selectively alter that stamp based on who's sending.  

The main goals of that footer are to be reassuring and informational, without being too obtrusive -
 1. Notifying recipients that your email was delivered securely
 2. Provide additional information if recipients are not familiar with Paubox (optionally clicking on the link in the footer for more information, this helps eliminate confusion and questions)