From your Account Page, locate the Take Assessments Block. 

If you have already Shared a Course/Book with an Instructor, you will see the "My Shared Course" Button at the top of this block.

Click the My Shared Course button to reveal all of your courses that are currently shared.

Locate the book/course you want to affect, and Click the Unlink Button.

You will now be prompted to start the “Share Your Course” process again - Choose your School, Teacher and Course Name.