Depends a little on where it is. But to dig it out is the best option, almost always. They have a tough root and rhizome system and you may need more than a spade. A mattock might be your best tool. Killing it with a weedkiller is possible, but you'll still need to remove the rhizomes, (the underground stems which will be gathering in force to emerge and replace older stems), so you might as well dig it up anyway.
Phil J