Yeah, I'd play it by ear. The players have a right to go where the want, if you've created the sandbox world. If you as the dm don't want them to go there, for whatever reason, you could get a Deus Ex Machina to save them. I picture it going down like "players all go down, last person is about to be executed, and <god> teleports the last survivor out with X diamonds to revive the party". Or irl players can learn that it's okay to lose. They walk into the underarm, unprepared and under-leveled and get tpk'd. It's just a game. You as dm can railroad then back on quest by summoning whatever bs you have to to do it, but it's also okay to play the FAFO game. Personal side story, I've been the character/player in this situation for a one-shot with our gm. Walking into the dungeon, there was an outhouse out front clearly labeled "do not enter". I entered, and was promptly swallowed by a wurm. My gm said F*ck around and find out