You can unsubscribe to the newsletters in Airmail easily by choosing the Unsubscribe option.
You will find the option as shown in the picture below
You can add the Unsubscribe button in menu buttons and it will appear on the top of preview panel for quick access.
Please go to Airmail presences > Actions > Menu buttons 1/2/3/4 > Unsubscribe
If the messages do not have the unsubscribe option available, Airmail asks you to bounce the message.
Bounce function sends an automated message informing the sender of the message about a delivery problem.