You cannot remove (delete) any of OOo’s predefined styles from a document or template, even if they are not in use.
You can remove any user-defined (custom) styles; but before you do, you should make sure the styles are not in use. If an unwanted style is in use, you will want to replace it with a substitute style.
To delete unwanted styles, right-click on them (one at a time) in the Styles and Formatting window and click Delete on the pop-up menu.
If the style is in use, you receive a warning message.
|| Make sure the style is not in use before deletion. Otherwise, all objects with that style will return to the default style and retain their formatting as manual formatting. This can be a problem in a long document.
If the style is not in use, you receive a confirmation message; click Yes.