[KeepFlying]

Not a bad thought. Though I think it had some problems that they already considered.

They mention not wanting to overstate the danger ("touch this and you die") in case someone touches the rock and does NOT die, putting doubt on the whole message. I think that poisoning the area creates a similar risk.

If you poison the area then you run the risk of a future civilization cleaning up the obvious poison at the surface, thinking that the message refers to only that poison. This would leave them thinking that the area is safe after their cleanup.