I don't think that you can delete it yourself. What you can do to keep that assignment from showing up on your "To Do List" is to give it a grade of "EX" (excluded). Just remember to keep track of whether the student completes all of the assigned work later on in the course.

Also, remember that you can keep students from accessing later modules and assignments by not publishing them. You can also set the availability dates so that students are not prematurely submitting work.