Eryngiums die right back, so if you want to remove spent flower heads then cut back right to the ground. Many people however prefer to leave them, as they provide attractive interest during the winter months.
Don’t forget to add it to your ‘plants I have’ list to receive regular care instructions: [LINK]https://https://www.shootgardening.com/plant/search?q=Eryngium+planum&search_submit.x=0&search_submit.y=0[/LINK]