I read this short story back in the late 70’s / early 80’s in middle school or high school. It was a dystopian futuristic tale in which society mounted some sort of “speakers” that influence the masses into consuming more goods. One individual tries to convince the others of what’s happening, but only seems to begin to persuade a few. At the end of the story, the man climbs the pole to tear down one of the “speakers,” but is subsequently killed. The few people (of whom he nearly convinced) look on in horror(?), only to be distracting into “realizing” they need to buy more random products (imbued into their minds by the mounted “speaker”).
As Danny suggests this does sound like The Subliminal Man by J. G. Ballard.
There are signs that deliver subliminal advertising messages. This is discovered by a man called Hathaway who tries (in vain) to convince other people of this:
"The signs, Doctor! Have you see the signs?"
Frowning with annoyance, Dr. Franklin quickened his pace and hurried down the hospital steps toward the line of parked cars. Over his shoulder he caught a glimpse of a thin, scruffy young man in ragged sandals and lime-stained jeans waving to him from the far side of the drive, then break into a run when he saw Franklin try to evade him.
"Dr. Franklin! The signs!"
Head down, Franklin swerved around an elderly couple approaching the outpatients department. His car was over a hundred yards away. Too tired to start running himself, he waited for the young man to catch him up. "All right, Hathaway, what is it this time?" he snapped irritably. "I'm getting sick of you hanging around here all day."
At the end of the story Hathaway climbs a sign to sabotage it.
Suddenly Franklin stopped, the sense of relief fading instantly. With a jolt he saw that several of the police on the ground were armed with shotguns, and that the two policemen climbing the sign carried submachine guns slung over their shoulders. They were converging on a third figure, crouched by a switchbox on the penultimate tier, a ragged bearded man in a grimy shirt, a bare knee poking through his jeans .
Franklin hurried toward the island, the sign hissing and spluttering, fuses blowing by the dozen. Then the flicker of lights cleared and steadied, blazing out continuously, and together the crowd looked up at the decks of brilliant letters. The phrases, and every combination of them possible, were entirely familiar, and Franklin knew that he had been reading them unconsciously in his mind for weeks as he passed up and down the expressway.
BUY NOW BUY NOW BUY NOW BUY NOW BUY NOW
NEW CAR NOW NEW CAR NOW NEW CAR NOW
YES YES YES YES YES YES YES YES YES YES YES
Hathaway falls to his death after the police fire a submachine gun at him. The story ends with the protagonist Franklin accepting there is nothing he can do.
"Hathaway was right," he said simply.
"Was he? Advertising is here to stay. We've no real freedom of choice, anyway. We can't spend more than we can afford; the finance companies soon clamp down."
"You accept that?" Franklin went over to the window. A quarter of a mile away, in the center of the estate, another of the signs was being erected. It was due east from them, and in the early- morning light the shadows of its rectangular superstructure fell across the garden, reaching almost to the steps of the French windows at his feet. As a concession to the neighborhood, and perhaps to allay any suspicions while it was being erected by an appeal to petty snobbery, the lowest sections had been encased in mock-Tudor paneling. Franklin stared at it numbly, counting the half-dozen police lounging by their patrol cars as the construction gang unloaded prefabricated grilles from a couple of trucks. Then he looked at the sign by the supermarket, trying to repress his memories of Hathaway and the pathetic attempts the man had made to convince Franklin and gain his help.
He was still standing there an hour later when Judith came in, putting on her hat and coat, ready to visit the supermarket. Franklin followed her to the door. "I'll drive you down there, Judith," he said in a flat voice dead voice. "I have to see about booking a new car.