Assassin will stab someone's hands.
Elementalist can burn the weapon so that its boiling hot that you have to drop it.
Warrior twist someone's wrist.
Mesmers make the weapons seem to become a snake or something that the wielder will drop them.
Necro uses some soul twisting mind bending skills that will fall under soul reaping that makes the wielder drop the weapons.
monk will chant something to make the wielder regret kill so many people that they drop the weapon out of guilt
dervish swipe their scythe to steal the weapons
paragon shout (a very loud one) that sends a wave length that shock the weapons to fall.
finally ritualist will send a spirit to steal the weapons.

imagine you are fighting fighting fighting, suddenly you find yourself fighting bare hand.
off course, you do not loose your weapons, it'll just be disable and you will find your character without a weapon for a duration of 120 second or a suitable time frame.