I just take the eggs right out from under mine, literally lifting them up sometimes. A few times they got angry with me and made this horrendous growling noise, and once I leave with the eggs they start "screaming" at me. Just take them from her.
They're greedy little buggers when it comes to eggs, they don't just want their own egg, they want ALL of them! My girls will all use just two of the 8 laying boxes I have in that coop. I guess they have favorites seeing as there's only 12 hens in that one. Just the other day I went down, and low and behold, there was one of the broody girls sitting on 7 eggs! LOL!
I don't bother trying to break mine because eventually I would like to have a rooster or two, so it would be nice if some of them would hatch out some eggs for me.
I have heard that you can separate her in a raised crate or cage for a few days, with no bedding, so she won't nest in there. You can keep it in your coop, if you have the room, and just release her back into the flock when you're done trying to break her. I personally have never done this, because like I said, I want some roosters eventually. But I heard it works.