I do acknowledge that there are indeed players willing to buy that much GM but really it is rare and usually very pitiful as that means they dropped a huge amount of money that not everyone is usually rolling in to spend on such a game. Sure it sucks if you lose to an otherwise terrible player spending hundreds of dollars just to compete, but such instances obviously are few and far between.
Setting a GM spending limit seems the most logical solution. I mean, if they're willing to spend that much I'd imagine they'll immediately resume their spending after 24 hours, so its not that revenue for the devs is lost, it's just spaced out. He'll probably still buy 6 million GM , but over the course of a few weeks, not in a day or 2. Also, gives good players some breathing room from just being overwhelmed from an instant spam when they actually occur.