find out what brand and if possible,what model,then see if you can download
the installation manual for it from the manufacturer's site,and that should
have the data needed to uninstall it. If it has a starter disable,you may
have to resplice a few wires. Chances are the control module is under the
driver's side somewhere,possibly cable-tied to a bracket.

An alarm is only useful if YOU are around and able to respond to it in
time,and prepared to deal with the thieves(IOW;armed).
No one else is going to bother checking on your car.

Good choice - if it's got a starter or ignition kill or some other sort
of immobilizer, simply cutting it out may disable the car altogether.

Best suggestion I'd have is just to take it to a car-stereo shop and ask
them to remove it.  Should take them no time at all and cost only a
small service charge... some may even do it for free (or a tech may do
it free on his own time) if you offer to let them keep the alarm.