This is a good catch Stalker. The rules seem to indicate that the characters can do this. Technically, dropping a weapon is not a 'power', so you might go for a very literal interpretation of dominate and disallow it, but I'm not certain if this is true. In your case you can't even make this argument, since throwing a weapon is one of those 'everyone has it' powers.
Even if it is allowed, I say it is imbalanced. Regardless of whether a power is a daily or not, disarming many opponents is equivalent to defeating them. It is an unnecessary game mechanic that is not balanced for 4e. If you consider that attack value probably drops by 2 and damage drops substantially without a weapon, it simply isn't right. If the dominated person is a character with a magical weapon, this becomes uber imbalanced.
How should you respond to your creative but troublesome player? Let them get away with it this once, and make a clear ruling that domination cannot be used in this way in the future.
And also, thanks for the skill challenge system.
I'd allow it, just to see the player's face the next time his character gets dominated and he has to throw his favourite magic weapon into a bottomless pit.
Cheers,
Roger
No offence, but I avoid this solution like the plague. From the player's point of view, they are being punished for innovation. If my PCs used a clever idea on some monsters, and the next batch of monsters suddenly and independently came up with the same idea and used it against the players, they would think to themselves "Gee, I better stop trying to use my brain, the DM doesn't like that!"
When my players find a new way to exploit a power or ability or even the rules that upsets balance I usually:
1: Reward them, usually by letting them get away with it for this one encounter
2: Fix the problem, usually with a house rule. In this case I would rule that dominating can only use movement and actual attack or utility at-will powers. The player cannot command the monster to drop his weapons, strip his armor, or lock himself in a cage.
This way the players are given a small reward for thinking, and the game doesn't have to be ruined in the long run.
When your players decide not to push a monster off a cliff because they are worried that other monsters will suddenly start pushing them off cliffs from now on, that's metagame and not really fair to the players.