自分では使ったこと無いけど
http://unity3d.com/support/documentation/Components/class-Rigidbody.html

Constraints Restrictions on the Rigidbody's motion:-
Freeze Position Stops the Rigidbody moving in the world X, Y and Z axes selectively.
Freeze Rotation Stops the Rigidbody rotating around the world X, Y and Z axes selectively.