Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
If you have a code for the alarm system, enter it, and press End or press STOP or press ENTER (most likely that last is on the keypad.)
If it still sounds, or if you have no code, you need to figure out which circuit breaker in your home, will turn Off the alarm system box. Flip that breaker, listen for the alarm no longer sounding, leave it Off by the breaker switch until the technician arrives.
That's about all you can do - aside from cutting wires. That's usually not easy to do, nor easy to figure out... it is a security system after all, not easy to disable by any casual intruder. And to instruct you as to what wires to disconnect or to cut, I would really have to know the maker and model #... So the above, I hope, will help, in the meantime.