I agree that her W should probably just completely nullify dashes while active, particularly since any dash beyond the first is a matter of not paying attention to her zone even after she blocked one already. In fact, I wouldn't mind if she lost the damage on her W for that, since the latter is never going to be that amazing (it doesn't scale with the stats she builds, its overall damage is alright but never as good as a single instance of Q damage, and W gets leveled up last, so it usually remains at 70 for most of the game).