How do you clear all item drops in Minecraft?

If you're trying to clear all item drops in the world, you can run:

/kill @e[type=item]

which will kill all dropped items in the world. However, there are slightly better ways of doing this. Namely, only clear items within a radius of x blocks from the command block. This preserves drops elsewhere, but still clears the first item:

/kill @e[type=item,r=x]

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Additionally, if you want to ensure that something can only be triggered once, place an input stabilizer on it:

This device will "lock" the redstone state to ON, even after the item is removed. It cannot be re-triggered unless you manually break the redstone.

You can expand on this by only making it hold one pulse if so needed.