GameDoc
Explorer
There is a level 17 fighter power that disarms the target, but there are no actual mechanics for doing so outside of using a power. It's basically way too overpowered of an ability to continuously use, but the DM could use page 42 if needs be.
If you did want to use that particular fighter attack as a template, it's from the PHB1:
Exorcism of Steel (Fighter Attack 17)
You chop at your foe’s hand, causing a grievous injury and forcing him to drop his weapon.
Encounter
Martial, Weapon
Standard Action Melee weapon
Target: One creature
Attack: Strength vs. Reflex
Hit: 2[W] + Strength modifier damage, and the target drops one weapon it is holding. You can choose to catch the dropped weapon in a free hand or have it land on the ground at your feet (in your square).
This is more than a simple disarm attack, so you could make a weaker version without rendering the original power obsolete.You chop at your foe’s hand, causing a grievous injury and forcing him to drop his weapon.
Encounter

Standard Action Melee weapon
Target: One creature
Attack: Strength vs. Reflex
Hit: 2[W] + Strength modifier damage, and the target drops one weapon it is holding. You can choose to catch the dropped weapon in a free hand or have it land on the ground at your feet (in your square).
I would basically go with the Strength vs. Reflex and on a successful hit, there is no damage, nor can the attacker snatch the weapon as it falls; the target just drops a weapon on the ground.
Of course, fair is fair. Monsters who use weapons may employ the same tactic.