I've actually left a game because a GM did (1), amongst other things. I'd consider doing the same with a GM that did (2). Neither of those are a respectful way to treat your players, not to mention friends.
Here's what I'd recommend if you go with a power. Note this is a rough design but hopefully something folks could use as a springboard for something more polished:
Disarm
At-Will * Weapon
Standard Action Melee Weapon
Target: One creature
Special: This power triggers an OA.
Attack: Strength - 2 vs. Fort OR Dexterity - 2 vs. Reflex
Hit: The target drops it's weapon at it's feet.
Notes:
(a) It takes a minor action to pick up a weapon.
(b) You suffer a -2 penalty to the attack roll if the weapon is being wielded in 2 hands.
(c) you suffer a -2 penalty per size category the target is larger than you.
Now all you need to do is:
(a) Have the monster's weapon proficiency
and
(b) Have the monster's unarmed attack damage
calculated.
Likely, if it's an existing weapon at least, it should be 2 or 3 proiciency and the weapon's damage should be appropriate (with size adjustments factored in), so the unarmed would drop to xd4 (again, size adjustment factored in), assuming there aren't claw/bite/etc attacks already on the character sheet.
Of course, rarely does a monster have a magic weapon. He doesn't rely on expertise and magic to get his bonuses to hit. So, a disarmed monster may have his d12 weapon dropped to d4 weapon and have a -2 to hit (or d10 dropped to d4 and -3 to hit), but a 15th level PC losing his +4 greataxe would lose 8 (2 for proficiency, 4 for magic, 2 for expertise. Not to mention dropping not just down to a d4, but also losing +4 to damage rolls, etc). Of course this just talking about normal melee classes.
Disarm a rogue, and most of his powers stop working. Disarm a ranger, and most of his powers stop working. Disarm a barbarian and most of his at-wills stop working. Certain classes have restrictions tied to the type of weapon you are wielding ... losing your weapon severely hurts you in that regard.
Part of the issue is that monsters and NPCs are not PCs. They work differently. Luckily for the PCs, a monster taking their weapon would only get a small bonus to their hit rolls and damage (there are rules for equipping monsters with magic items).
Up thread there was a MMO mention of their "disarm rules". It seems like a temporarily loss of the weapons use, reflected by doing less damage and it being harder to hit. BUT, you get it back after some ammount of time, your opponent doesn't get to use it against you, nor take it and run away resulting in the permanent loss of your item. That is the exact kind of thing being suggested here, things causing the opponent to take a penalty on their attacks and/or reduce their damage. Which, is only a status effect that partially does a disarm, but eliminates a lot of other things.