I don't really do many paver jobs but what about talking to them about the situation and explaining to them that just about anything you put into the joints will be blown out every time you clean.
Maybe something a little more permanent like an epoxy or grout to fill in the joints? It can be colored to look like sand. That way after it has cured some, it should take the pressure and then no more re-sanding. Otherwise you are looking at re-sanding after every cleaning and have to charge them for it.