People are curious and adventurous. And greedy, power-hungry, and short-sighted. It doesn't even work today to warn people of something dangerous.
As has been said many times, any warning sign, particularly pompous ones, may always be interpreted as a sign of worship instead. So just pile a few hundred thousand skelettons on top -- a literal sign of death. We probably cannot do better. If anyone in the future does not understand this when digging it up, then a few people will need to die until they do. I don't think there's a solution.
Except maybe not to produce dangerous material that lasts longer than human memory. But that's, well, you can read the first paragraph again.