From what I know of my Vectra, are you unlocking with the key? Using the remote should unset the alarm. Failing that, search the car manual, and failing that go to a Vauxhall agent.
If you lock with remote,unlock with remote. If you lock with key, unlock with key. If there's still a prob, it may be a central locking switch/servo fault. My Carlton done this also, the fault was driver door servo not fully moving.
calibra not on remote locking?
alarm is the car horn?
You need to remove the drivers door card and remove the white switch that surrounds the lock cylinder and press oune of the switches to de-activate the alarm, leave the white switch off and your alarm will never turn on again