If they arent, then they can throw objects through a basic circle but it wouldnt bring it down. But, as others pointed out the sorcerer could still do the exact same thing.
If free will was a factor, it isn't the fact that rocks thrown by ghouls don't break the circle, it's the fact that ghouls literally choose not to throw rocks through the circle in the first place. It's so ingrained in their psyche that they can't break a circle so they would never try.
The circle (assuming they had time in the middle of a combat to make one) should have been a 'block against spell casting'. In which case, nothing would be hindered by it except spellcasting. If it was a 'block against attacks', then any attack could break it.
I wouldn't allow a sorcerer to break the circle for free on the same round he cast. At the very least, he'd have to make a maneuver to break it. Then, on his next turn, attack. Or, even if the ghoul had the free will to break it, they'd need to beat a difficulty.
If the ghouls had no free will, then the ghoul should receive a FP against its High Concept to keep it from doing something to break the circle. (since it has no free will, it can't turn down the compel)
It wasn't who broke the circle that was the issue, or necessarily even how ... it was that the circle was broken at all. Yes, they seem to think of it like Harry's shield bracelet and no amount of cajoling on my part could get them to accept otherwise. I cited multiple examples from the books...hell, I even read them the passage from TC with Binder to underscore their error... and was talking to a brick wall
Mechanically though,
a block is a block is a block.
If the block could be bypassed by throwing a pebble through it, that's a compel on the wizard who created it, IMO. Maybe it's a compel on her blind spot Aspect, maybe it's a City Aspect. But to completely bypass a block without a roll should be a compel, I think.
But, a circle certainly isn't a full-proof safe zone. It's a block.
Edit: I see why the players were upset and why they think the mechanics are bad. The mechanics DO take these kinds of things into account: compels. So, I think they are mistaken on that matter.
That being said, I think everyone would have been much happier if you'd offered the Wizard a FP before you blasted her. Then she'd have had the choice to take the full damage or keep the circle. If she chose the former she'd have a shiny FP.
In any case, having the choice would have smoothed things. (Speaking of Free Will)