I'd tend to do it as a maneuver (probably based off of lore, but conviction could work too) to set up an aspect.
And then, if something that doesn't have free will at all tried to cross the circle, I would (as the GM) compel that aspect against them - disallowing them from passing, but giving them (or their master, if appropriate) a fate point. This could also apply to a mortal mage trying to use magic across a circle boundary (if they were in some way prevented from simply breaking the circle directly.)
For things with, well, let's call it "partial" free will (such as powerful fey), they'd have to spend an action trying to break the circle first (their conviction vs. whatever skill was used to set up the circle).
And for a pure mortal? That aspect just doesn't apply; they can probably break it with a supplemental action, or otherwise go about their business without caring that you're in a circle.
This is the thing with aspects: they're only as powerful as the GM and players make them be. In a game I'm running, one of the PCs conjured up a fog to foil a gunman; by base game mechanics, that just gives them one free tag on the "fog" scene aspect; a plus two bonus on one defense roll. Not very satisfying. So I compelled the aspect against the gunman (preventing him from even rolling to attack, but giving him a fate point)... and then turned around and compelled the aspect against the PCs, too. Made the spell feel a lot more powerful. This is the same sort of approach I'd use with a circle.