Broody every summer

Some hens go broody others don't. You can't prevent it. Your only option is to get rid of the one.

 

Or be prepared to break her everytime she goes broody. Are you separating her completely, or just off in the coop somewhere?

 

Breaking her right in the coop might help, if you've got room.
Letting her out once or twice a day might help too, if you've got the time.

If you don't want her to hatch out chicks, best to break her broodiness promptly.
My experience went like this: After her setting for 3 days and nights in the nest, I put her in a wire dog crate with smaller wire on the bottom but no bedding, set up on a couple of 4x4's right in the coop and I would feed her some crumble a couple times a day.

I let her out a couple times a day(you don't have to) and she would go out into the run, drop a huge turd, race around running, take a vigorous dust bath then head back to the nest... at which point I put her back in the crate. Each time her outings would lengthen a bit, eating, drinking and scratching more and on the 3rd afternoon she stayed out of the nest and went to roost that evening...event over, back to normal tho she didn't lay for another week or two.
Water nipple bottle added after pic was taken.