Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
For this sort of (severe, bothersome) problem, there are some things you can try:
1. if you were left with a User Code, or some other code, for this system, you can try entering the code. See if that ends the alarm status / armed status, if you enter the code number followed by END, or followed by STOP.
2. if you were left with no code, and if you have any patience with the siren and alarm, you can try entering a default Engineer Code for your system.
3. if the first two options are no-go, or not good for meeting your needs, we can tell you how to permanently (and with only a slight destructiveness and with no risk to your safety) disable the alarm system permanently.
Let me know what you think would be best to try for. Thanks.