i've tried the 'operational reasons' thing but then they wanted to know why etc etc (i think they knew when they requested the exit row that they would prob be too fat and were hoping they wouldnt get noticed)
now i simply say.. "i'm sorry but we cant fit extension seatbelts in the exit rows, its a company policy. i do however, have a great seat here for you up the front. can i help you with your bags?"
And give them no choice to argue.