There are various ways.
There is the a -> b collision (dis|en)abling, which, btw, isn't the same as b -> a.
Then there is the "global" (ok, it's per set) disabling. And that's for anything -> anything, including phaser beams.
Ok, here is how you do it:
First way: pA.EnableCollisionsWith(pB, 0)
And you better also do:
pB.EnableCollisionsWith(pA, 0)
To enable them again use 1 instead of 0.
Then there is the following way:
pA.GetContainingSet().GetProximityManager().RemoveObject(pA)
Or, and this is what I think you want:
pA.GetContainingSet().SetProximityManagerActive(0)