Deleting an Enumeration Type 
You can delete an enumeration type only if it is not being used in an if-then rule, decision table or rule script.
Locate the Project Resources node ( Where is the Project Resources node?) and double-click the node.
In the Project Resources editor, choose the Enumeration Types tab.
In the enumeration type editor that appears, choose an existing enumeration type in the table and choose the Delete button.
In the dialog box that appears, choose OK.
Save the changes.