Sorry to say, but you're screwed any way you look at it. You don't do anything, those guys most likely will not pay, you place their players on waivers or redraft and the owners in the lead will be pissed while guys near the bottom have a new chance they do not deserve.
My advice, freeze the guys who didn't pay (not that it will make a huge amount of difference to them most likely) and find a way to make them pay, either by freezing their teams in other leagues that you are in with them (if that luckily is the case), or in some other way. If all else fails, beat the shit out of them
They committed, and it seems like a pretty serious pool at 50 bucks a piece (especially in American dollars, if I assume correctly!). At least teach them a lesson if they refuse to pay...
The lesson that you should learn from this is do not let people into your pool until they pay; that way you can avoid this whole mess next time