In Monty, it's possible to fire an automation action when a contact unsubscribes from your marketing messages. 

To setup, simply browse to Settings -> Workflow Automation then : 

  1. Either add a new automation or edit an existing one
  2. Then all you need to do is select 'Email Unsubscribe' in trigger dropdown. 

Note : this will fire on all open deals that is related to a contact.