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I'm sorry to hear about the problem. It sounds very severe and bothersome.
To turn the system Off, you would need to remove power from it. That may be difficult to do without damaging the system, e.g. by cutting the power wires - these systems are designed to be not-too-easy to disable.
We could approach the problem another way: First of all, disconnecting the battery will add a 'power failure' alert to whatever else is going wrong with the system.
Second, determining what error code is showing on the system when it goes off, could help settle the problem. Tell me any error code you see on the system box, even now, and I will advise you as to what it means and what to do about it.
(Otherwise: cut power wiring to it. It will not be easy to fix, but it will disable it. Alternatively, deprive the klaxon or other 'sounding' device of power, so that it will go off but not make noise. Or, at very least, muffle the klaxon or other sounding device, if you cannot figure out how to disconnect its power. Note that removing batteries will not help - they are only backup batteries in the event of power failure - though if you cut power wires, you will also need to remove the battery after that.)