Does Stun reduce Alpha Strike damage to 0?

No, Stun does not reduce the damage Alpha Strike deals to 0.

According to Rule 423.1.b, "A Stunned Unit does not contribute its might to damage in the combat damage step." Because the Stun condition specifically restricts combat damage, it has no effect on damage dealt by card effects like Alpha Strike.

Furthermore, per RiftJudge FAQ #9251, a stunned unit is still able to deal damage when directed to do so by a card effect (such as Challenge or, in this case, Alpha Strike) because the restriction is limited to combat.

Summary: Stun prevents a unit from dealing combat damage, but it does not prevent that unit from dealing its full might as damage when commanded to do so by the Alpha Strike spell.