I don't think that you can turn off 1-Click, but...
To avoid it, you might try the following:
In the menu bar, go to Store > View My Account (your_username)...
and in the bottom section where it says "Reset all warnings for buying and downloading music" press Reset Warnings. This might help you from accidentally clicking a purchase button.
Also, if the problem is not with accidental presses, but you'd rather use some sort of "shopping cart" then you can create a new playlist with the title "Shopping Cart" and then drag items from the iTunes Store into that playlist to make up your mind later. This is also what the "Wish List" feature is for.
