Elymus Magellanicus out of control

I hope you can help. We redeveloped our Park two years ago and created new flower beds, as we are just yards from the beach it was difficult to choose plants that with survive never mind thrive. One that we were recommended was Elymus Magellanicus (although what we have is green rather than blue). To say it has thrived may be an understatement, it is taking over, creeping into the turf and threatening to suffocate everything else.

The ones we were supplied are clearly the rhizome type and these have spread beneath the weed matting and even pushed up through it. If there is any advice you can offer us to control it or even get rid of it entirely we would be very grateful.