I realise this is on a Jeep rather than a Disco, but...
The Disco has something very similar - it buzzes if you have the door open with the keys in, and if you leave the lights on. I wanted to keep the latter but get rid of the former, so I clipped the wire from the ignition switch which goes live when you put the key in then used a bit of heat shrink tubing to insulate the bare wires. Result: I can sit in the truck with the keys in and the door open without annoying noises, but I still get a warning if I leave my lights on!
I wouldn't stick a screwdriver through the buzzer for this reason.