Ultimately, if they choose to do this, there's little that can be done to stop them.
My preference, stepping back for a minute, is that they therefore don't play in any English based competitions alongside it. I know this would necessitate a "correction" in finances at all clubs outside the 6, including us, but the chasm between those 6 and the rest would be such that I think it would slowly kill the rest of us anyway. At least if we were all playing on our own level, we could run the game accordingly. I suspect they'd only choose to leave us anyway, in the fullness of time, so I'd rather they were just told to **** off now and let the rest of us get on with it.
As a side note, there would have to be an adjustment to budgets even if they did stay in the league, as the Premier League is definitely going to become a less attractive proposition to TV companies anyway, so that will be the case regardless (albeit on a different level).