Surely written warnings, if some future society discovers them, they might be curious enough to have decrypted at least one of our present day languages/methods of communication.
Just have the same concepts relayed in as many languages/ways as possible, and then make the site sufficiently difficult to infiltrate that it would take a sufficiently advanced civilisation to break into it.
You could even tier the messages, and use words that would likely be common and thus more likely to have been recognised based on discovering whatever other shit we've left around.
DEATH.
THE THINGS HERE MAKE DEATH.
THIS MATERIAL WILL KILL YOU.
And progressively more complex and complete messages, etc translated into Chinese, Spanish, Braille, French, pictograms, what the fuck ever.
And if they're too lazy/careless to try and decrypt any of the fucking obvious messages, fuck 'em.