I think you have two basic choices. One, do as Aart does, create a pen where the hen cannot leave and the other chickens can't get to her nest. There are different variations to that and may involve moving your broody. If you decide to go that route get back and maybe we can discuss some details.
The other option is to leave her where she is. If you do that you need to mark the eggs (I use a black Sharpie) so you know which ones belong. Then every day after the others have laid check under her and remove any that don't belong. As long as you remove them daily you can eat them without finding anything strange from the incubation in them.
Bad things can happen either way but both are usually successful. If you move a broody she may break from being broody. If a hen with the flock comes back from her daily "constitutional" (leaving the nest to eat, drink, and poop) and another hen is on her nest she might go to the wrong nest. I have that happen maybe every other year but I just toss her back on her real nest when I notice. There are other things that can happen.
Good luck!