Dropping 20 plus ton flat rocks on them would do the trick. Sop up anything that oozes out from under the boulder and then carve their name etc into their very, very large headstone.
Nah.
Twenty ton millstones. Crush them then grind the corpse to a pulp. Do not immediately clean it of blood and gore. Let it get ripe. Carve the executed persons names, dates, and offenses into the exterior circumference of the stone.
Periodically, send it on tour to every jail and courthouse in the state to educate the public as to what ultimately awaits them if crime seduces them.