We get this... a LOT. Your alarm system is beeping or the siren is going off and you just bought the house or don't have the code.
The following information is provided as-is and without warranty and we do not accept any responsibility for anything that may happen to you or your home as a result of following this information. If you are not comfortable please hire a pro to pay you a visit. We assume you are the legal home or business owner and not a thief who just wants some peace and quiet...
With that out of the way...
Most alarm systems consist of a keypad in the main entry area and then a metal enclosure in the basement, electrical room, wiring room or closet that houses the brain and a battery back-up. If you need to shut down the alarm you need to disconnect power and the battery backup.
Look for a power wire coming from the metal box that goes to a plug-in AC power transformer. Unplug it. If your power transformer is hard wired to the side of a junction box or your electrical panel, you can disconnect one of the two wires leading from the transformer to the alarm panel. DO NOT TOUCH the 110V electrical wiring!
Then open the metal enclosure and you should find a battery with two terminal connections. Disconnect one of them.
At this point the alarm system should have stopped making noise.
Note: If this is a professionally monitored system there can be a tamper switch inside the metal enclosure that will alert the monitoring center that the enclosure has been opened.