I was sat in the car while waiting for about 90 minutes today, and managed to turn it on at least 5 times

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If I do anything about it, I'll probably interrupt one of the wires to the seat switch with a rocker switch on the centre console somewhere, which will leave it working as normal most of the time until i want it disabled. That way others can drive it without needing to know about the modification.
I believe the plug you want is the larger of the 2 white ones under the seat (at least on the SE model), and hope it is a simple switch that is normally open. I can check that in the wiring diagrams.
The likely sticking point for me, is finding a matching plug and socket so that I don't have to cut any wires.