Rigidbody Momentum Cancelling

Is there an easy way to cancel all momentum that a rigidbody has?

Use rigidbody.velocity and set it to 0, like this:

function FixedUpdate () {
    if (Input.GetButtonDown ("Jump")) {
        rigidbody.velocity = 0;
        rigidbogy.angularVelocity = 0; 

You will also want to set the rigidbody.angularVelocity to zero if you want the rigidbody to stop completely, including rotation. Note that this may result in unrealistic behavior, however, and use it sparingly.