To delete a part from the parts palette, select the part on the parts palette and select Modify Palette > Remove Part from the Options menu. After you confirm the deletion, the selected part is deleted from the parts palette.
To delete a category from the parts palette, select the category on the parts palette and select Modify Palette > Remove Category from the Options menu. Confirm the deletion, and the selected category and all of the parts in it are deleted from the parts palette.