A lot has to do with the voices only they can hear in their heads.
With some broody hens you could strap them to a rocket, shoot them to the moon, and right after they crash-landed in a crater they'd get right off and start incubating some moon rocks (right after taking a nice moon-dust bath). But other hens are more easily discouraged.
The others have given excellent advice. All I can think to add is to give them fake eggs, or just a couple real eggs, or both, to settle on before swapping them for the eggs you want them to hatch. My broodies are bantams, so I don't mind sacrificing a few banty eggs to see for certain if they'll settle in a new location. After 1-2 days (maybe longer for those hens of yours) I figure it's safe to swap in the standard-sized eggs I want them to hatch.
Otherwise, you've found yet another great use for duct tape.

With some broody hens you could strap them to a rocket, shoot them to the moon, and right after they crash-landed in a crater they'd get right off and start incubating some moon rocks (right after taking a nice moon-dust bath). But other hens are more easily discouraged.
The others have given excellent advice. All I can think to add is to give them fake eggs, or just a couple real eggs, or both, to settle on before swapping them for the eggs you want them to hatch. My broodies are bantams, so I don't mind sacrificing a few banty eggs to see for certain if they'll settle in a new location. After 1-2 days (maybe longer for those hens of yours) I figure it's safe to swap in the standard-sized eggs I want them to hatch.
Otherwise, you've found yet another great use for duct tape.