Best to give a vague, non-committal reason so there's always a chance they can talk with you again in the future.
I'd talk to the OP again, after I make it bigtime and roll up in the Maserati, with a 6' tall, red-headed supermodel with a Scottish accent on my arm... and I'd say "fuck you, dude" and then buy a round of beers and sit back and laugh about the whole thing.
Of course, that's just me... :-)