Mate you must be crazy if you seriously think a union ban would even be contemplated by the unions themselves, and even if such a thing was in place do you think it would be taken remotely seriously by the wannabe fraternity?
In fact the only time a union recruitment ban has been seriously tried in recent years was with Cathay and HKALPA, one of the most unionized airlines in the world, and the net result of the ban was it achieved nothing, hordes of people kept on queueing up to join their airline under the ban, the union recognized it wasn't achieving anything, the ban fizzled out, and the power of the union in that airline was all but broken as a consequence, certainly membership numbers are much lower now than they were beforehand.
An industry wide ban across an entire country / continent is just a daydream, especially at a time like this when union power is pretty much at an all time low, half the airlines in the world now don't even have union recognition, in fact such a move would only be playing straight into the hands of people like O'leary in my opinion.
Nope the only thing that will stop the rot, is when the inevitable happens, some captain becomes incapacitated and some bloke who's paying to sit in the RHS spuds a plane into the ground. Then we'll see the end of pay-for-training in a right old hurry I would imagine. The regulatory authorities don't really care about pilots crying over their eroding T&C's, but they'll move pretty damn quickly when there is a post-fatal-accident media circus.
In fact if BALPA ever do decide to take this issue seriously, and if I was the person in charge of determining the strategy, rather than attacking my own power base of future members, I'd far rather start a targeted campaign towards a few chosen influential media bodies, highlighting the safety shortcomings of the current system, where those with the money get chosen over those who are most experienced and suitable for the job. There are plenty of newspapers who love to run a good aviation scare story, turn them loose on it, release a list of airlines that operate pay-for-training schemes and those that don't, and let the public decide.