cut and pasted from a post by SAJ77 ive done so it must be easy (called flapper mod if you want to do a search)
"Its simple to do, at the right hand outlet, under the car there is a black plastic circular actuator with a rubber pipe going to it. Simply remove this pipe and block it using a large coarse wood screw - screwed 10 or so mm into the pipe ensuring the head of the screw blocks the pipe fully. Replace the pipe back into actuator - job done! The actuator basically is now not functioning and the flap will remain open."