In your section Assignments list, locate the Shared assignment that you want to stop sharing and select Stop sharing.
 
Check the section(s) you want to stop sharing the assignment with and click  stop sharing. Click  OK in the confirmation window that appears. Connect will provide a confirmation message that the assignment is no longer being shared.

Best Practices:

It's easy to stop sharing an assignment you have shared with a colleague. When you stop sharing an assignment, your colleague can no long see the changes you make to it (questions, policies and more).