You can delete an item from your collection in Mediathread.
Once you log in, go to the right side of your screen and click on the button View Full Collection.
Once the View Full Collection button has been selected, the following screen gives you access the Full Collection. Filter for your items to items that are owned by you.
Look at the item you want to delete, in the bottom left corner there should be a "garbage can". When you click on that garbage can, it delete's the item.
Note: when you delete an item, it is considered an orphan, it still exists in Mediathread it is just not discoverable or viewable by your classmates or instructor.