Making non fixed particles act as if they are fixed

[Corey]

I want to make it so that when something runs into an object that is
not fixed (pseduoFixedObj) that the pseduoFixedObj experiences no
forces from the objects colliding with it but the colliding object
behaves as if it ran into a fixed object. Are there any built in
methods/members/properties for this? If not, can anyone suggest how to
do it.