You cannot remove (delete) any of Calc’s predefined styles, even if you are not using them.
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.
Replacing styles (and then deleting the unwanted ones) can be very useful if you are dealing with a spreadsheet that has been worked on by several people.
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. Select Yes in the confirmation pop-up.