There is no Enable/Disable checkbox for Rigidbody2D componentin the inspector

Steps to repro:
1. Create a new unity project
2. Add a sprite
3. Add the rigidbody2d component to it

Note that there is no checkbox to enable disable the component while all other components have it.

Resolved(By Design): We purposely don't allow this because not only does it have non-trivial performance characteristics but it also leads to confusing implicit behaviour of associated colliders.

What should colliders do?

Any collider will look for a valid Rigidbody2D on the same GameObject or 'up' the parent GameObject hierarchy. Disabling a Rigidbody2D means that the existing colliders will become static i.e. not attached to a valid Rigidbody2D or worse, attach to another Rigidbody2D somewhere up the hierarchy.

For this very reason, there is the following property: http://docs.unity3d.com/ScriptReference/Rigidbody2D-simulated.html

This allows you to stop the Rigidbody2D and all its attached colliders from interacting in the physics simulation. It is also super fast and allows for the typically required behaviour of stopping the rigidbody and its colliders from working.