how do i turn off alarm?

I have a frontera 'p' reg, everytime I put the key in to turn the alarm off, it activates the alarm, once the horn has stopped the lights keep flashing for about 2 mins, once they've stopped I try again but the alarm just starts again, as it has a immobiliser I cant start the vehicle, so instead of being in work today I am sitting here trying to find out how can I at least knock the horn or alarm off. Thanks