Welcome to Just Answer, It is not really possible to have this disconnected as the system has a back up using the horn and indicators, it would also have implications on the vehicles insurance.
At the end of the day it is designed to be as difficult as possible to deter thieves, the siren can be removed but the indicators and horn will go off as said.
This is a common fault on both and the Astra and Zafira 9 times out of 10 it is due to a faulty power sounder (siren) , it has in built re chargeable batteries and these only last a few years and means the power sounder will need replacing.
To be sure the alarm system has a diagnostic system built in and you will need to have it tested on a diagnostic computer to get its fault codes read , you don't have to go to a main dealer as most good garages have this kit.
Let me know if you need more information.
I hope this helps, best regards, Vauxdoc