No way to block them without taking the bumper off then you might as well change them, you may be able to get to the washer lines if you take the passenger side arch liner out and block them off there but I can't remember 100%.
i put 1 way valves in mine never had a problem since
thats itI guess it would stop the flow of air back up the pipe so the liquid wont go any where.
Won't ever be the same if i take the bumper off, does the xenon washer have a different pump to the windscreen ones? If so can i disconnect it?