They'd have to buy the GM version first.
Otherwise I doubt anything would stop them, just like there's nothing stopping folks running a traditionally published adventure from getting a copy of the books and/or PDFs. (Or sneaking a look behind the GM's screen during a break.)
I agree with Parody on this view. IF your concern is to limit your "prying eyes" of players, frankly there isn't a failsafe. Just as others have said.
Even now there is nothing keep a player from going to the local game shop or half price books or amazon and procuring a copy without your knowledge.
The most sure fast way I have found in 30 years of dealing with this is to not be so "tied to whatever canon" you utilize that your players can do this with impiety. There are those that suffer with, what I would label, "PC gamer-ism" that buy the game and then the cheat book to beat the game sooner. They follow those same matrix for Roleplaying.
Since our tables' campaign is within Greyhawk (the first, longest running, and best), there is a mountain of "canon", "semi-canon", and "Fanon" but that doesn't make any of it gospel to me. I have info from the original writers that extend 10's of years ahead of our current campaign date, but that doesn't force me to use any or all of it. I have often changed it via discussions with them on what they would do "different"..
There are also several places that the PCs have changed events, or I simply didn't like what was originated (example Gygax had some canon around a wrecked space ship with laser, and hand grenades, to me just didn't fit) .
On a differing note, and I must admit, I take a certain level of ZEAL at the looks on those "metagamer / PC gamer's" when it doesn't play out as they have "read". I have broke my players from this habit you describe with differing monsters, traps where they were not indicated, for just a few variables.
And then not all my orcs are CR1 single hit die creatures either... hehehe
From a RW perspective, I have my own campaign and then have compartmentalized others based on canon dates.. If/ when the market place comes into being, I will migrate those for other GH faithful to manipulate in their own way. BUT till we know how that mechanics of it will play out I don't honestly waste a lot of time/ effort on it. why work it two or more times until Rob and the gang get their footing on how they will manage that new piece of the RW puzzle?
As I tell my players, feel free to google, read, etc all you want, I credit that to increasing their player knowledge and supplementing their skill checks, but just because Gygax wrote it that way doesn't mean that is the only way.
