While not named on Supernatural, this symbol associated with Hell appears on a number of occasions. The trident or pitchfork, appears to be both symbolic and a practical - it can prevent a demon from de-possessing its human meatsuit.
Having fled his palatial home, Crowley has taken up residence in a ruined house, that has tridents painted on the walls and windows. When Crowley and Dean capture Brady in order to ascertain Pestilence's location, Crowley carves a trident into Brady's chest, telling Dean it will prevent the demon leaving the body.
Crowley: This bit of carving will tie our friend here down. No zapping off, no smoking out - locked in the meat suit.
When Sam and Dean find Lucifer in Detroit in an abandoned building, he doodles a trident into a frosty window.