All is said in the title, I find the move
1.d4 Nf6 pretty annoying even if it's probably not a great move. What are the suitable moves to "punish' this opening play? I add that I am rather a Benoni/Benko player.
If you're a Benoni player, there's nothing wrong with 2... c5, which will get you in familiar territory (3. dxc6 e.p. is harmless and not something White wants to play - exchanging a center pawn for a flank pawn).
While it's not a great move, Black isn't immediately better, just about equal. So it's dangerous to try to 'punish' this move, especially if White has probably more experience with this line than you (and less with the main Benoni/Benko lines).
The most direct way to punish this move seems to be to attack the d-pawn immediately. Black gets a very slight advantage. For example
[FEN ""] 1. d4 Nf6 2. d5 c6! 3. c4 cxd5 4. cxd5 Qa5 + 5. Nc3 b5! 6. Qd3 b4 7. Qb5 Qb6 8. Qxb6 axb6 9. Nb5 Ra5 10. e4 Nxe4
I'd recommend that you run this line by the engines. They do tend to agree that this line gives Black a very slight advantage.
I've known this player for years (at 2300+ strength) who frequently played: 1. d4 Nf6 2. c4 g6 3. d5?! in order to avoid the Grunfeld Defence. While the move is slightly dubious, him playing it obviously showed that the move wasn't very bad.
If you like the Benoni, 2...c5 is definitely an option. However, the kicker here is that White doesn't have to play 3. c4 and transpose to the main line. Instead, White could play 3. Nc3!? and if you play 3...e6 then 4. e4! could be unpleasant.
A solid option would be 2...e5. While this gets you out of your familiar territory, it does give you good play on the dark squares. Example:
- c4 Bc5 (or Bb4+) 4. Nc3 a6 (giving the Bishop a strong home on a7) 5. e4 d6, etc is for the most part just an improved KID (the regular "terrible" Indian Bishop is now your strongest piece... it is White who would really want to exchange dark squared Bishops now).
One last suggestion could be 2...b5. It prevents 3. c4, and if 3. Nc3 then 3...b4. Since White also can't play e4 for the moment, the d5 pawn is vulnerable. You can play Bb7 and e6 to put pressure on it and take it out. NOTE: If White plays a4, you would just bypass it with b4.
Wes's answer seems quite good but I think ...e6 is also a strong alternative. The point is that you just get pieces out, and d5 comes under pressure. It also seems that, since you play the Benoni, the exd5 capture will lead to positions favorable compared to the main lines, since White never has time for e4. You can castle and play Re8 (f3 is not really an option, as White's king knight will be terrible for a while). Your position will not be cramped. Here is what happens if Black tries to push pawns.
[FEN ""] 1. d4 Nf6 2. d5 e6 3. c4 (3. Nc3 Bb4) Na6 4. a3 Nc5 5. b4 Nce4 6. f3 Nd6 7. e4 Nfxe4!
- ...Bb4 is also good here, and maybe less risky, as Black can just threaten to exchange the bishop for the knight, again either winning a pawn or collapsing the White center. The two bishops aren't enough compensation for the loss of time from d4-d5-dxe6 or a3.
So 2. d5 looks annoying until you find a way you like to go against it. The main thing is to attack the pawn immediately and force White to trade it for a less aggressive pawn or waste time defending it. Fortunately, there are pretty straightforward moves to dissipate White's advantage, but there are more adventurous ones like Na6 too. So I bet after you see it a few times, you will have fun neutralizing it.