Hi @JuliaL ,
That sounds really frustrating. When you click on the assignment in the To-Do list, are you able to enter editing mode? Can you just unpublish it so that students can't access it anymore at least?
When you click from the To-Do list, can you get to the Speedgrader? Can you see actual submissions? What are students actually doing/submitting?
Have you looked on your Quizzes and Discussions pages to see if the assignment shows up there?
Have you looked at Pages to make sure you don't have one with the "Add to Student to-do" box checked?
Have you tried logging out of Canvas entirely and logging back in to try to "reset" your Assignments page?
Have you tried clearing your cache and making sure your browser is up to date (also to try to "reset" your Assignments page)?