I have a similar problem on my 2004 Subaru Forester. After talking with several mechanics, they said that the cheapest and easiest way to disable the alarm is to find the main fuse for the alarm and pull it. All the mechanics said that this is very risky though because if the fuse box is wired together a certain way, by you pulling the fuse for the alarm, you could mess up all of the other fuses in that box, essentially causing your car to fail. It's a risky thing to do, but from what the mechanics said it's and easy way to solve the problem, you can even do it yourself if you have a fuse pulling tool, or even with a small/pointly pair of needle nose pliers.
alex, May 2008