1.  Open the Assignment Editor by going to Assignments, My Assignments on the navigation bar to the left, then clicking on the pencil to the far right of the assignment name. 

2. Click on the Settings Gear at the top right of the page, and then click on Open All Settings.

3. In the far-left column, click on the Control Retakes tab, and then check the box to the right of "Disallow Assignment Re-takes."