subscriber
A person who signs up to receive messages from a particular client and/or their list(s).
There are 2 ways this can be done. At the bottom of any campaign, there is an Unsubscribe link that will automatically remove the user. You can email them and tell them to select this to remove their address.
You may also delete the user yourself in the Control Panel. Go to Search Subscriber, enter the persons address who contacted you and then search for the user. In the results page, you can then delete the user from your list(s).