That's not the point. The point is toe A) discourage the silly exploit by enforcing common sense rules...
Just for the heck of it, I will observe that the problem does not appear if one grants common sense a role.
Echo!
Needless to say, I agree with Ariosto... A lot of game rule problems (not jsut this one, and not just in this edition) dissappear with a small dose of common sense.
Hmm, but there is tear gas, and flash bombs.
...and 2) reinforce it by presenting an alternative that works similarly, but is more in line with the rules and balance of the game...
...Blinding Bombs from Adventurer's Vault, for example...
Whaddya know... you're right.
Tear gas (Blinding Bombs) and flash bombs (Jolt Flasks) both make appearances in
Adventurer's Vault. Similar items were not uncommon in earlier editions.
Anyway...
As already suggested, I think part of the issue here isn't so much that the PCs want to do something cool with what they find, but that original adjudicated effect was too powerful for what it is.
It's not an easy thing to manage on the fly in the middle of a game, but some consideration has to be taken when ruling on the effect of little environmental tricks like this. The trick either needs to be locational (triggering a rockslide in a cave already weakened by an earthquake), situational (shoving an enemy in front of the 6:21 Cross-Town Express just as it passes by), or not much more useful than an At-Will (tossing sand in someone's eyes to inflict a -2 penalty on attack rolls until the end of your next turn).
There are a lot of other ways to make the very minor tricks useful, without making them better than character powers. For example, the type of action the character gets to use is a big one.
If, for example, you rule that flour in the face doesn't deal any damage but inflicts a small penalty to attacks for a round by using a move action, then it will get used occassionally but not constantly. The move action encourages players who don't need to move much to use it when they can. At the same time, it prevents them from using it constantly, because sometimes they will need to move around and any standard At-Will power will be more powerful than what they get out of the flour.
On top of that... It's okay to admit that you got it wrong and tell the players, "Look, I know you want to carry flour packets around to blind people with, but honestly... I made a mistake, and that effect is really too powerful. If you want to use cheap dust packets, that's fine, but I need to change their effect. Otherwise, here's a list of more expensive alchemical items that can blind enemies."