Stacks will not let you delete a partial from the stacks library if the partial is still in use. This is just a safety measure in case you have a partial somewhere deep within your document that you didn't realize you were still using. However, Stacks will allow you to hide a partial from the library by using the "Remove from Library" command. If the partial is still in use, then the partial will come back to the library on the next launch of RapidWeaver. If the partial is no longer in use anywhere in the document the partial will be gone forever.