You must delete sheet drawings from the project using the Explorer View on the Sheets tab of the Project Navigator. When you delete a sheet drawing, all sheet layouts in the sheet drawing are deleted from this project. 
- On the Quick Access toolbar, click Project Navigator 
.  
- Click the Sheets tab, and click 
 (Explorer View) in the title bar.  
- Select the sheet drawing you want to delete, right-click, and click Delete. 
 
- In the Confirm Sheet Delete dialog box, click Yes.