It's the old saying of the "death of a thousand tiny cuts". If it has a number, it can be killed.
So these things don't get a number. If the wizard player says "Hey, I do a gigantic 50 shift ritual to find the thing", it's absolutely valid to say no. It's behind a circle that blocks out all magic, it can't be found by magical means, you'll have to find another way (here's a fate point for your trouble). The same goes for supernatural beings trying to break a circle. The circle keeps them out by magical means, so they can't just use, for example, their superior strength to break through it. They may be able to will themselves through, but they'll often not have nearly enough power there as anywhere else.
Basically, you don't have to create a "block:X" circle, you can simply create a magic circle aspect and reinforce it's narrative impact. If the novels say anyone can draw a circle to cut off magical energy, that's what they do. If a spirit wants to enter the magic circle, he can't, cause he's a spirit. You wouldn't let a human roll on gnawing through a concrete wall, because it's just not going to happen. I see this situation pretty much the same.