please help! optima alarm system?

I moved in my house and wasnt even aware we had an Optima Xm alarm.
it was until we had a power cut i realised. the fuse in the fuse box which is responsible for the main lights, is also connected to this alarm. so when i flick it bk on, the alarm also comes on. i do not have a code for it.
it now means i have no main lights.
how i can i disconnect the alarm? cut out battery perhaps? please help!