C# overriding past velocity

I want to know how to override some velocity from a Rigidbody permanently, until the target touches the ground.

Problem : OnCollision2D is too slow, and my override that is based on OnCollision2D is too, making the override impossible (it's an "under second" thread).

Any ideas ?

P.S : the code is long and 3 scripts are involved. So I'll only insert code that is necessary, for your own comfort.