Just disarm the weapon and toss it over a cliff and that's it for the rest of the encounter if he has no backup weapon. That is just way too powerful compared to any existing powers.
Which is why Solos you fight on a cliff should have some forced movement resistance, or be able to fly, or something similar. The point is that reducing someone's attack and/or damage for the rest of the encounter is too powerful an effect to be included in an at-will, just like any effect that lasts until the end of the encounter. If you can disarm someone that easily, the only solution will be to have the solo carry half a dozen weapons, or give him natural weapons instead... And you can guarantee that that'll annoy your simulationist player, especially if he builds his character to do disarms all day.Or, you know, push him over the cliff and that's it for the rest of the encounter.
There are a lot of ways to resolve disarms. The DM has to resolve it appropriately for the group he's in. But to say that disarms are somehow not a part of D&D is a really strange statement.
No, not really strange at all, BECAUSE IT ISN'T. Replace "my character wants to disarm" with "my character wants to do called-shot neck and decapitate with every swing." Guess what, SAME ANSWER, it only works when they're at 0 HPs.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.