Under game constants under the "advanced" tab...there, you can change the rate of corpse decay and bone decay. Set them both to a few seconds and the units/corpses/bones will pretty much dissapear a moment after they die. -VGsatomi
You can also do it with object editor ---> units ----> choose the unit you want and roll down till you see 'Combat - Death Type' and choose 'Cant raise does decay' or 'Cant raise doesnt Decay' im not sure you will need to check them both..