I have ideas but you have to get down to the horn's wires and then track down its relay. Or maybe the switch in the steering is broken and is in "always on" state.
Just for fun, my horn did the contrary of yours, it stopped working. I replaced it 2 times with used ones from identical scrap cars. They broke too. Then my best friend gave me his '76 Oldsmobile horn. People are very impressed to hear such a sound from such a small car.

This old horn has been running flawlessly for 3 years.