Weapons of Mass Destruction
"The use of atomics on human beings shall be cause for planetary obliteration."
-The Great Convention, Dune
"You maniacs! You blew it up! God damn you. God damn you all to hell!"
-Planet of the Apes
Atomics, bioweapons, nanophages, antimatter conversion, artificial black holes- there are any number of ways to reduce a planet, it's inhabitants, or selected portions thereof to their constituent stardust and mop up aterwards. The problem is that ALL of these things are gigantically illegal. If you try to pull a stunt like this without gathering massive political and social support (by fair means or foul,) you will most likely prompt immediate and terrible military retribution from other star systems- every other noble house or faction will put aside their differences and pound on you instead.