I'd go with something like the Forbidden Zone signage.
http://cdn.collider.com/wp-content/uploads/planet-of-the-apes-forbidden-zone.jpg
No matter what they come up with, someone will always want to look inside.
It's an interesting thought experiment to follow down the rabbit hole.
Those symbols look foreboding. But human curiosity is a major obstacle to overcome.
Or not human. They are looking for a universal language for any being who doesn't have cultural reference. Something like the golden record, I guess, which I would never figure out.
Poop. Nobody likes poop. Except a few weirdos but they need to die anyway. So make it a gigantic pile of poop.
Not a gigantic toilet. Kids will climb it just to flush it.
I'm not sure what you eat, but if your pop doesn't decompose after a few weeks. Perhaps you need to reduce the amount of iron in your diet.
I mainly posted this so that when I'm sobererer I can dig more into the subject. I researched it breifly like 4 years ago.