2 possibilities, if it is a factory alarm system, try pacing the key in either the driver's door, passenger door, or trunk lock cylinder and turning it to full unlock. If that does not turn it off, then you have an aftermarket alarm system. Most aftermarket alarm systems are disabled when they "sense" the ignition is in the"on" position. If you disconnect one of the battery terminals, put the key in the ignition and turn it on, then re-connect the battery terminal you will disarm the system until you turn the key off again. This should give you enough time to either take it to a shop to have the old system removed or disabled. You can take a shot at it yourself, you should find an alarm control box under the dash, probably near the steering column. Good luck.