Couldn’t we shoot it into or towards the sun? I mean after a certain distance gravity would take over and pull it right in. If we shot it for a direct hit on the sun, it would probably burn up, and even if we missed, it could still just orbit around the sun inside Mercury’s orbit. And if it isn’t cremated after getting to the sun, who would be able to go and get it? No one for at least 10-15 billion years. And if they are that patient, then good for them.
Except for the political/religious types who wanna build a radiological weapon.
Wrap it up in duct tape, drive out into the desert, venture a few miles away from the road, bury it.
Put it in a plain wooden box, properly serial-numbered and registered in a nondescript government warehouse, right next to the Ark of the Covenant.
Give it to Jimmy Hoffa, c/o Judge Crater.
Set the controls for the heart of the Milky Way galaxy.
Give it to The SCO Group, and then have them build a trial based on the presumption that they have it.
The whole subduction zone idea is probably a very good one, but magma seems to get recycled every so often. Humans may well be extinct before something placed on a zone now comes back to the surface, but why tempt the sapient cockroaches? Best to mark it Raid or Rid-X first.
Well, you could put it up Rosie O’donalds’…
I probably shouldn’t finish that. 
The subduction fault will keep it out of human hands for many many years, Shooting it into deep space like the pioneer or voyager probes would work OK. As a last resort a landfill should work for maybe 100 years, but after that time things burried might be worth enough for people to do landfill mining and dig up the trash in search of anything valuable.
Purolator.
Donate it to the National Archives.
With any luck, it’ll be years before they even file the thing.
I’ve been thinking…would entombing it in aluminum be better?
Concrete & cement crumble & dissolve from ordinary salt.
But soda cans can be obtained easily, & melted with ordinary charcoal for fuel.
Aluminum doesn’t rust or corrode. It doesn’t crumble with exposuire to salt. It is slow to erode.
Put it in a 200 lbs block of aluminum, out in the desert.
It’s too cheap to be valuable, too sturdy to easily crack open, & too common to be worth hauling miles in the desert heat for such a poor return.
Well, what’s our budget for getting rid of it?
If you’re Bill Gates or the President of the United States or something, launching it into space might be a pretty surefire way of getting away from human hands. (By launching it into Venus, the sun, Jupiter, or just out of the solar system.) Though there’s always the danger of an accident or failure during launch sending The Box back down to Earth, but you could probably safeguard that a bit by encasing it in a durable capsule that you could relocate if it got back to Earth. (If it’s going to end up in the Sun, or somewhere in Jupiter, you don’t have to worry about it having high-vis paint and a radio beacon, I wouldn’t think)
If you’re on a more limited budget, things get harder.
The Marianas Trench, with a maximum depth of about seven miles, would be a safe place to drop it, naturally, but if you’re on a shoestring budget, just getting there might be difficult. (You’d probably be willing to go into hock for the sake of protecting the world from the Evil Box, but depending on your financial situation, you might not even be able to do that.) Perhaps a cheaper alternative, if you live by an ocean already, would be just to find a fairly deep stretch of ocean off the continental shelf, with nothing of industrial (e.g. fishing, oil deposits, etc) or scientific value (deep-sea life, undersea vents, methane ice deposits), take a boat out to it (you’ll probably want an expert to help you with this, if you don’t have a good amount of sailing experience), and dump The Box overboard in a block of concrete.
If the ocean’s out of the question (like if it’s Posiedon’s Evil Box, or something), maybe you could hide it amid a lot of nuclear waste, like the Yucca Mountain facility, or inside the Chernobyl power plant. The latter, although not as secure as “buried inside a mountain filled with nuclear waste and guarded by U.S. forces,” is probably easier to sneak into, and The Box probably wouldn’t be in danger of being removed, anyway. (“Hey, Yuri, want to take a cinder block from right next to the melted reactor slag?”)
How about dropping it into a live volcano. I saw that in a movie once.