When
    you delete a custom item type, all items of that type are also 
   deleted. You can not undo this action.
You must be the page group administrator, the portal administrator, or have Manage privileges on All Page Groups to delete a custom item type.
In the Navigator, click the Page Groups tab.
Navigate to the following path:
Path: Page Groups > PageGroupDisplayName > Custom Types > Item Types
where PageGroupDisplayName is the page group in which the custom item type was created.
Note: If the custom item type is shared, it will be in the Shared Objects page group.
In the row for the custom item type that you want to delete, click Delete.
In the confirmation page, click Yes.
You can not delete base item types.
   Creating a custom item type
   Changing a custom item type's basic properties
   Adding attributes to a custom item type
   Adding procedure calls to a custom item type
   What is an item?