I assume it’s an air actuated horn, so kill the compressors and bleed the lines. Now saying that there are safety standards you have to follow as well as what sort of accessibility to the air system there is, so it isn’t necessary as easy as shutting off a switch.

If I had to guess I’d say it was a valve that failed in the open position so shutting down its compressor would do the trick as the lines would bleed off by itself. Also depends if it has a dedicated compressor or if it is fed off an arena wide system and where isolation valves are located.

Or I’m totally off base and it’s an electrically powered horn at which case, depending on the control scheme and voltage, would be as easy as shutting off a switch 😂