Well I'm sure there have been enough attempts to stop Look Inside using different defensive strategies provided at the moment. But with the current defense, apparently there is no "absolute" tactic (whether there is an absolute tactic is another question) to defend Look Inside well.
In any defense (and offense for that matter), something has got to give out, and it's a matter of minimizing whatever that has to be given out. And apparently Look Inside is one of the more effective tactics that allows maximum damage to the opponent's defense (in another words, opponent has a hard time trying to minimize the thing that has to be given out from their defense).
I suppose the problem lies in the question of this thread itself...
A well played 2-3 zone can stop a LI.
The thing is there are not many teams who can play en effective 2-3 but there are many who can run an effective inside offense.
I would change some words;
The thing is there are not many teams who can play an effective 2-3, but there are many who can run a 3-2 with so great performance and without the need to have special players.
Yes, and that's not because 3-2 has something special or it is very easy to play. That's is because the players needed to use it effectively are on the market. Sadly, those same players happen to be suboptimal to use an outside offense. So you have this scenerio where most guards on market can easily defend with 3-2 but cannot make a nice outside attack, then m2m can deal with most outside attacks when 3-2 it's just devastating against them.
Training patters are the key to understand current situation and very likely, the solution to it.