I always use diluted muriatic acid to remove rust stains from masonry, but it's a dangerous chemical that can cause burns, blindness and lung injury if misused. It's also known as hydrochloric acid.
There is a product called Bar Keeper's Friend that looks like scouring powder, but it contains oxalic acid, which removes rust stains and is not as dangerous as muriatic acid. You might make a thin paste of it, glop it on and brush it around before rinsing it off. Don't get it in your eyes.
There's also a product called Iron Out that comes in several forms, one of which is a spray-on gel that seems made for this application.
Needless to day, test on an inconspicuous area before attacking the whole stain, and see about re-routing the boiler overflow or you'll have another stain next time the boiler relieves itself.