Answer by Skylinerw for How do I kill mobs using /kill without them dropping loot?

The DeathLootTable string tag can modify which loot table the mob will use when killed. The "empty" loot table will cause it to drop nothing:

1.12 and lower:

/entitydata @e[type=Zombie,r=40] {DeathLootTable:"minecraft:empty"}

1.13:

/execute as @e[type=zombie,distance=..40] run data merge entity @s {DeathLootTable:"minecraft:empty"}