It does take a toll on their overall condition.
They lose a lot weight and their comb and wattles grow pale. They also pull out a lot of their feathers on their underside and breast.
I just broke broody on a hen who hatched babies for me a little over a month ago. I usually let them do their thing but with the heat, I don't want additional stress on them if I can prevent it. I just scooped her up, took her to the hose and gently wet down her underside every time I caught her camped out on the nest. It only took 2 days and approximately 4 soakings a day to break her of it. I also shut the door to that nest. She climbed all over looking for a way in but came to terms with it.