Yes, they can. As required by the CAN-SPAM law, every message sent includes an unsubscribe link to allow the recipient to opt-out of further mailings.
This link is placed in the footer of your message automatically as each message is sent, and it is a "smart" link. The recipient can simply click on the unsubscribe link and they will receive a confirmation number for their unsubscribe activity.
You can monitor this activity, along with many other activities, by selecting Reporting, then List Activity Summary or List Activity Detail report and entering beginning and ending dates.