To delete a form that came in the mail (for example, W-2, 1098, or 1099) it's best to do it from that form's Summary screen. Search for the form number (for example, W-2) and select the Jump to link in the search results to get to the Summary screen.
You can't delete Form 1040. That would be like yanking the foundation out from under a building. If you want to get rid of the information you've entered, you can clear your return.