The full list of undocumented features for mail message, controllable with context:.

    MailThread features can be somewhat controlled through context keys :

     - ``mail_create_nosubscribe``: at create or message_post, do not subscribe
       uid to the record thread
     - ``mail_create_nolog``: at create, do not log the automatic '<Document>
       created' message
     - ``mail_notrack``: at create and write, do not perform the value tracking
       creating messages
     - ``tracking_disable``: at create and write, perform no MailThread features
       (auto subscription, tracking, post, ...)
     - ``mail_save_message_last_post``: at message_post, update message_last_post
       datetime field
     - ``mail_auto_delete``: auto delete mail notifications; True by default
       (technical hack for templates)
     - ``mail_notify_force_send``: if less than 50 email notifications to send,
       send them directly instead of using the queue; True by default
     - ``mail_notify_user_signature``: add the current user signature in
       email notifications; True by default

You can do this via Settings -> Technical -> Email -> Subtypes.

Uncheck the checkbox "default" for your action to disable the messages.

Note: this is for version 7, idk if they changed this.