Content Type Deletion
You cannot delete a site content type if it is being used as the basis for other site or list content types. You must first remove this content type from all lists that use it and delete all child site content types that are based on it.
You cannot delete a content type from a list if that list contains items of that content type. Windows SharePoint Services does not consider items sent to the Recycle Bin when making this determination. If those items are restored after their content type is deleted from the list, those items are assigned the default content type for that list.