It looks to me like Carex pendula (a sedge) which is sometimes grown as an ornamental plant. It's a bully and I've noticed becoming very widespread in the wild in recent years, I rip out any which appear in the hedgerows around my allotment. If this came from a planted trough it's possible the grower would have used it as a plant to give height but in my view it's a pretty dodgy nursery/garden centre which would use such a beast.
It is difficult to tell from a photo of a trashed plant but I go for either this or Cortaderia (pampas). I doubt you'll kill it with any chemical a member of the public can legally buy and I'm doubtful commercial growers will have anything available. I'm sceptical about covering it with anything as this will be powerful enough to grow through a barrier. Your best bet is to dig out all you can and physically damage what is left sufficiently to prevent it from growing back.