Like I said before... telling someone to drop their cargo and have it destroyed is an act of terror and not a piracy attempt.
it is up to the player if they want to go that route or piracy to steal
You should not make rules to regulate this. This should rather be common sense. It is just too bad the community members sometimes lack it.
Ex:
-Lane hackers stopping an Ageria with some goods they want to be destroyed.
-Xenos stopping a non-liberty ship in liberty.
-Gaians stopping OSC , planetform, etc ship that are carrying things they do not like.
All of these examples would be instances that the "drop all your cargo" makes sense.
For a regular pirate to do that, they would have to be making some RP justification.
So if you really want to add a rule for this thing.... add that the dropping of cargo to be destroyed act needs to be justified in the player's RP.
Edit:
And not just having: "I dont like you drop cargo or die"