Contact the security company that installed the system and have them disarm it.
They will charge for the visit.
To the rest of you – this has happened in the past :
A company goes out of business and leaves the building they were in, but does not remove the alarm system. The power does not get turned off because the building owner wants the lights on when showing the building to the next tenant. How is the building owner supposed to disable the alarm system without destroying it? If the owner understands electronics, it’s easy to disconnect the alarm bell and the telephone connection, but most do not. They want the system ready to work as it helps to sell the building to the next tenant.