Upon death disable the spawn2 of that NPC spawn2
Edit: Probably would have to UpdateRespawnTime of the new spawn2 before enabling it so it doesnt just respawn instantly
Also have it enable a spawn2 of a clone NPC that doesnt have that item on its loottable.
Seems like an easy-ish way to do it
Could also script the drop with Perl and a qglobal as well