You could pull the horn fuse, that should kill it. The horn fuse is hot all the time. Two possibilities, one is security system, the other is problem with horn switch or wiring in the steering column.
The horn switch is ground for the control side of horn relay. Part of key fob security horn beep, is also a ground wire. In the diagram, you can see the two wires, The one for fob, take it loose, see if that stops the horn sounding by accident. Don't leave any live wires hanging loose, use electrical tape or something to insulate. I don't know if taking that one wire loose will cause any problems or not, I guess you can try it and see?
Of course, you still want the horn switch to work normally.