I think I've missed the boat on training my dog "food refusal" too.
She's pretty good at "its yer choice" but anything she finds for herself on the ground - she thinks is fair game. So I really ought to work on more "its yer choice" with yummy bits of chicken or chips at the park... sigh. She's quite good about things around the house. She will sit, drooling, looking at the open bucket of food and just bark... but she won't steal it. Not entirely sure how I got that. Enough stealus interupptus plus rewarding the drooly sit and look I guess.