Easy.
If you're absolutely certain that this occurence is the only reason for the current problem you simply explain said occurence to them in simple terms.
If they still, having made it clear to you already that this is the one gotcha, won't entertain your application, then explain to them in even simpler terms why their airline is no longer of interest to you as a prospective employer.
I suspect it is an excuse
If it isn't just a way of fobbing you off and they genuinely won't employ you because of this one thing then do you REALLY want to work for such a mob of idiots?
Just imagine how they will react when you ram another bird in one of THEIR shiny jets...stupid crunts will go appoplectic.