It might not be that simple.
It wasn't on my 2002 Focus with the separate key Fob, but I had to open it up to remember why.
On the 2002 - there aren't really button contacts - the rubber button cover activates a little microswitch on the PCB below it. Now - with enough skill, you could break or desolder the switch and remove it which would solve the problem, but you can't easily "put something non-conductive across the contacts". (Although, if I were trying it again, I might be tempted to see what SuperGlue or ThreadLock did on the switch when it was not depressed).
I did something different on the 2002 Focus and it worked well for me - I found the vehicle schematics and simply cut the wire from the Central Locking Module to the horn. This might not do exactly what you want. (I sometimes accidentally hit the button and my hazard lights flash, but the horn doesn't sound. The Anti-Theft would also not sound the horn. Also, if I press Lock twice, the parking lights flash but the horn DOES NOT sound.) Personally, the first thing is a minor annoyance, I don't think anyone pays attention to car alarms anyway, and I consider the last item a bonus.
I'm not sure if Ford still uses a hard wire to the horn anymore - so much is data-bus driven now, but if they do, that should work for you.