Well, you _could_ do it if you had first first placed a tie or constriction band at the base of the penis, just below where you planned to cut -- like a cock ring, but tighter, to the point that it became a tourniquet. That'd keep the blood in and the cock rigid until you released the tie.
Alternatively, you could do something that would vaporise the guy out from under you, but disintegration tends to involve a heat source. and that might cauterize the stump, so you wouldn't get the same effect.
(I seem to have put a disturbing amount of thought into this.)
