If you know that you are getting ready to play someone that plays the London System exclusively, even the London System God, Cyrus Lakdawala, would agree with me on this one and he has mentioned before that it's the one case where the London System can actually give White a clearly inferior (not even equal) position.
It's the Modern Defense!
1.d4 g6 2.Nf3 Bg7 3.Bf4 d6 4.h3 Nc6 5.e3 e5! and now:
A) Best for White is 6.Bg3, keeping the tension. With the possibility of an eventual f3 to swing the Bishop over to the Queenside, this is better than going all the way back to h2.
B) Most common, and bad, is 6.dxe5 dxe5 7.Qxd8+ Kxd8! and now the Bishop has to move back anyway as 8.Bg5+ f6! 9.Bh4 is even worse for White. Black has ideas of h5, g5, etc, expanding and making White's light-squared bishop totally useless, he's for all intents and purposes playing down a piece! The symmetrical position should be no means bother Black!
d6 puts a nice buzz kill on any London/Colle/Stonewall stuff. If you start Nf6 regardless of your opening you can usually get d6 in without deviating too far out of your own zone.
I like to out-bore these players. Piss them off. They have one plan and have no clue what to do when you shut them down
Yes this. I usually play the hedgehog and I'm having good games. The position is equal so you don't get advantages out of the opening as black, but white has no plan. It must be so frustrating for them.