When you get this error message you can take the following action. Create a text file using Notepad. Enter the following text exactly:
Save the text file as debug.ini
Place this file in the installation folder for your Revit version, typically:
C:\Program Files\Autodesk Revit "version" 2010\Program
When you run your software you'll get a message box confirm the debug mode "Allow deletion of Non Empty Worksets".
After making a backup of your project for safekeeping open the project and try to delete the item in question again. You should find that Revit will allow you to delete it. Hopefully this will help.