Agreed, I sometimes tuck my seatbelt round the back of the seat and plug it in when I use my 5 point harness to stop the warning. If you really wanted to you could probably short the pins in the multiconnector under the seat to convince the car the seatbelt is always on.
Be careful as depending on your cars spec and age unplugging the seat connector can trip an ECU alarm. I have heard that this can then either light a CEL or in some cases fire the Airbag. I am not sure about the latter but it is plausible I suppose. I know alarms do light when messing with seatbelts.
Best just to be safe and wear the belt
