On the other hand, having a foe throw his melee weapon at you is usually a shock. So if the thrower managed to gain a LoS, I'd allow him to impose a 'Surprise Opponent' Combat Manoeuvre (I just made this one up) forcing the target to not be able to act on his next CA. This would give the thrower a chance to draw another weapon, freely disengage or close in for an unarmed attack.