Short is just removing the system, all you can really do it put it in valet mode so it doesn't arm. That would only be a temporary "band aid" approach though. I would have the system either removed or replaced unless you can find what is triggering it.

Thank you - we don't use the term 'valet mode' here - what does that mean please?

I'm assuming this is an aftermarket alarm system that has been installed. All of those system have a mode it can be put into that prevents it from arming that is commonly used when you "valet" park your car. There is usually a small switch or button that is activated when the key is on and that suspends arming of the alarm.

Thank you - I'll explore - but I think the system is an OEM !

I guess that depends on whether it's a horn going off or a siren. It would have to be a horn to be factory and if that is the case, someone is going to have to determine which trigger is causing it.