If it's an environmental hazard, say they found a portal to the Abyss and want to assault Lolth (or any deity) in her fortress. Ask just once, "Are you sure?" A smart player will get paranoid about that and change his mind. If he doesn't, and promptly dies due to said environmental hazards, it's his own fault.

If it's an enemy like Big T, or a Balor, some good descriptive texts should work. Have Big T run over a town, then commence to start eating a mountain side while describing it.

I'm personally against the Worf Effect, especially if you included an NPC in the group just for this reason.
If you have to Worf it out, roll out the fight for the PC's, where they can see the dice. Level 4 characters, see how I just rolled a 6 on his first attack, and still hit over a 30 AC? Yes, I said first attack, then proceed to roll the overpowering encounters other 5 attacks all at once. That fistful of dice seems to unnerve players more.
AND THEN, you roll all his damage dice at once. 3d6+1d6 sonic+9 (str and magic) +20 power attack per hit. Guys, can I borrow some of your dice, I don't have enough. Then grab a calculator and make sure they hear you announce the damage.
If you pull a Worf effect solely with description (In box text, for those who are accustomed Living Greyhawk/Living Forgotten Realms), it won't have the same impact.