Not with Disarming Attack
Rules as written, Disarming Attack requires you to hit a creature, then it forces the creature to drop an item it is holding:
Disarming Attack. When you hit a creature with a weapon attack, you can expend one superiority die to attempt to disarm the target, forcing it to drop one item of your choice that it's holding.
The solar which released its sword isn't actually holding it, however, the trait description says that
If the hovering sword is targeted by any effect, the solar is considered to be holding it.
So it would work, if you did target the sword. Unfortunately, according to the feature description, you have to target the solar instead.
You can try another way
The trait description requires the solar to see the sword in order to control it:
If the solar can see the sword, the solar can mentally command it as a bonus action to fly up to 50 feet and either make one attack against a target or return to the solar's hands.
So, if you manage to blind the solar, which released its sword, it loses control on the sword, hence, becomes disarmed. Something like darkness spell should help. (Or an ordinary physical obstacle. Throw your sleeping furs on top of the sword. Thanks @Tommi for the idea)