Oh the number of times I've been 'stoned' electronically for this...
The DMG or MM utilized by a player at my table without prior permission is punishable by loss of XP, loss of hit points and possible expulsion for a session.
I have played since the beginning - and I have always stated that the Dungeon Master's Guide is just that a guide for the DM, not the players. I too play and DM, but most of my players do not. The Rule 0 is trying to put the horse back in the can... as I see it, if a player disputes your ruling and then cites from the book that says, you have ultimate authority to alter content they should immediately be struck by lightning for being too stupid to read the section that says...he's the DM, that's why.
I understand the need to regulate the 'big baby' syndrome of I can do anything and I'll prove it. But at the same time how many DMs have bent the rules to build a plot poijnt only to have Stanley T. Nerdburger say [whiny voice] umm, according to the DMG on page 12,145 the square of the angle of the hypotenuse is directly proportionate to the effect of El Nino on the hyperbole of the gravitic flux capacitor and therefore, you're wrong.. nyah nyah nyah [/whiny voice]
In 1st and 2nd edition at the very least I could say, "you have offend the gods, you are hit with 1 million, 1 millionth hp lightning bolts...take 1 hp of damage and make 1 million saves for all of your equipment", but now with object DR, and such, you can't even do that....
Granted MOST people do not try to unbalance the game with unfair knowledge; MOST people know when to separate player form character info, MOST people have the common decency to try to at least hide their copy of the forbidden tomes.... It is however the Lowest Common Denominator for which the no way, now how, no why rules existed. Frankly, I want them back!