There is not a tutorial, but if you go throught he general DataAssist tutorials and then give it a try, ask questions here when you get stuck and we can try to clear up any questions you have.
The basic steps are:
1) Create two pages: unsubscribe.php and unsubscribesuccess.php
2) on the success page, just put text that says "you have been removed from the email list"
3) on the unsubscribe page, create a recordset that is filtered by a passed in url parameter, probably the email address.
4) Then on that page add an update server behavior that updates the row returned from the recordset and updates an unsubscribe column... have it redirect to the success page after the update.
5) Add a link to your email for people that want to unsubscribe... it will be a link to the unsubscribe page that passes the email address as a url parameter so that the whole process will work.