The PhysicsMass.InverseInertia can be set to float3.zero - i.e. infinite inertia. This is the same as the freeze rotation option. Unfortunately there is no UI for this yet in the Unity Physics package, though if you look at the ‘2b6. Motion Properties - Inertia Tensor’ sample uses a demo script ‘Set Inertia Inverse Behavior’ that you might find useful.
If you want a Joint approach rather than manipulating the tensor check out [this post]( Unity Physics Discussion page-7#post-4460308).
We are looking into an adding the equivalent of the Freeze Position and Freeze Rotation options as full class options in the physics material but I don’t have a timeframe for that yet.