My advice is to blatantly favour 1 PC over the rest. If that doesn't work throw Pit Fiends and Great Wyrm Dragons at the party. Once you have beaten them all down near death have a couple of you DM NPC's come and defeat the demon/dragon/horribly powerful enemy and rescue the PC's from certain death.
If that doesn't work start blatantly cheating out in the open. When you roll a 1 or a 2, tell the player they have just been critted. Then roll on an old skool critical hit table to determine what body part has been severed/horribly disfigured/rendered useless.
If they still continue to say nothing, start making up new "house rules" each session. Apply these inconsistently and change them constantly, sometimes even mid-session. If anyone complains tell them that their PC has been eaten by a dragon/killed by a rock fall/died a horribly random death and that they need to roll up a new character.
If they still insist on playing with you start changing things up in the game. Start a new game each session (just tell the players you were bored of the old one). There are so many campaign settings out there why should you confine yourself to just one? And why use just one campaign setting per game? Mix them up! Play a Dragonlance Cyberpunk game with giant space hamsters as the BBEG's! Play a Dark Sun Forgotten Realms campaign set in ancient Greece but with laser weapons! The only limit is your imagination.
I hope my advice has been helpful for you.
Olaf the Stout
