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The best you can do for now under the circumstances may be to cut power off to the alarm system... or better, to whatever noisemaker is part of it. The siren/klaxon would be an obvious first thing to cut off, if it is sounding.
If the alarm is simply beeping, that's easy: cut the power to the alarm system box that is beeping. It's about all you can do without a code. Do you are least have a key to open the box? if so, open the box, remove the battery connections (probably not requiring a set of wire cutters, they generally unplug or undo), and then locate the power connecting wires... almost certainly they will have to be cut. Obviously, cut only one at a time, period. This cannot be emphasized too much. And, wear rubber gloves or use rubber-handled cutters.
Without a code for the system, that's about all you can do, crude as it sounds and is.
(If you can give me the exact model #, perhaps I can find a default code to try.)