Yes, at the end of each subscription-based message (sent from the COMMUNICATE TAB: group emails, newsletters, autoresponders), there is an "Unsubscribe" link that your contacts can click on and, in a new "Manage your subscriptions" summary screen, opt out of certain communications from you by un-checking the different types of messages and clicking on the button "Update my subscriptions" or opt out of all communications by clicking the button "Unsubscribe from all types of emails".
The unsubscribe link is not present in direct emails (sent from the client's Profile) or in work-based communications (appointment confirmations, reminders, session notes, client log notifications, etc.).
Also, the system does not send you email notifications when a contact unsubscribes from your group emails, newsletters, or autoresponders. This is to prevent spamming your inbox with numerous such notifications. To see the overall subscription status of your clients, you can run the "Subscriber List" and "Autoresponder Stats" reports in the REPORTS TAB or, for a specific client, you can look at the "Subscription Settings" section of their Profile tab (in CONTACTS TAB > Contact Name).