You can also disconnect just the button that's stuck on. This is very easy to do just removing the button/airbag assembly from the steering wheel.
The buttons themselves are pretty simple foil contacts. The airbag assembly itself is not designed to be serviceable but you can disassemble it. It's held in place by rivets which you will have to drill out. The yellow connectors on the airbag circuit are self-shorting when you disconnect them for safety reasons, so don't be afraid to work on the system.
All depends on what direction you want to go - disconnect the offending button, try fixing it, source another airbag assembly, route another horn button and mount somewhere on the dash... all up to you.