To remove an assignment from the Assignment report, click on the trash can button on the right side of the report.
You'll need to click REMOVE to delete the assignment from the report. The report showing the students who have completed the assignment will also be deleted.
You have the option to remove the skills from Mixed Review as well. Just check the box if you want to remove the skills and then click REMOVE. You'd check this box if the assignment was given to the wrong class or if you never expect the students to practice these skills again.