Here are the steps to stop the Pending emails from sending.
If the user's Email Access Type - Outgoing is set to Email Router, stop the Microsoft CRM Email Router and the Microsoft CRM Asynchronous Processing Service on the CRM Server.
If the user's Email Access Type - Outgoing is set to Microsoft Dynamics CRM for Outlook close Outlook on the client machine.
Run a CRM Advanced Find where Look For = “E-mail Messages”, and conditions: Status Reason = “Pending Send”, Subject Contains <specific to email you are looking for>
Delete the emails.
Restart the services on the server or Outlook on the client.
It is possible there might be some stragglers that get through but hopefully not to many.