On my Civic it's as simple as disconnecting the wire that goes to the seatbelt and carries the signal – may vary with other makes though.
It's a very silly EuroNCAP requirement – I always put keys in, switch the engine on, then seatbelt on, then drive off. But now they expect you to get in, pass keys to left hand, reach for belt with right hand, pass keys back to right hand, switch engine on, drive off. If you don't, BEEP BEEP BEEP BEEP BEEP. The ridiculous thing being if you do drive off with no seatbelt on, it shuts up and just displays a little reminder light on the dash instead. Stupid, stupid, stupid.