Disallowing Cube Rolling - Simply "Glides" along floor?

So I am creating a 2D game (Yes, using Unity3D) but I am having some trouble with Cubes. Right now I just have a cube that has a texture on it and is facing perfectly straight so it looks 2D, but when I push it, it rolls and can sometimes (If pushed hard enough) turn around and show it's real 3D nature.

What I want to do is stop it from rolling and disable it from moving anything but on the X axis and, if there is no floor below it, on the Y axis.

If you need an example just ask. I know I'm not that decisive.

For now, you have Rigidbody.freezeRotation. In Unity 3.2, more constraints will be exposed.