Yes, you can retire your Leanpub book or course any time. You can also un-retire it any time, if you change your mind.
A "retired" book can no longer be purchased in the Leanpub bookstore. However, there will still be a landing page for the book or course on Leanpub, which authors sometimes want to keep, for a variety of reasons.
To retire a book, please go here:
...making sure to replace YOUR_BOOK with your book's unique web address.
To retire a course, please go here:
...making sure to replace YOUR_COURSE with your course's unique web address.