I believe it turns off automatically after 3 minutes or so, but it is so loud that I couldn't imagine waiting that long. Try pushing and holding the PANIC button on the FOB, and if that doesn't work put the key in the ignition and start the car. That should do it. Disconnecting the main battery doesn't help as the alarm has a separate power source buried somewhere in the firewall area.

I don't know what year your roadster is, but if your alarm starts going off at random and you still have the original battery that came from the factory (some of them are 6-7 years old), that is a sign that the battery needs to be replaced. These cars do some strange things when the battery ages and the voltage starts getting low. I'm planning on replacing my 6-year-old battery within the next month, although so far I've had no problems.