decline the offer politely.
keep a pack of gum in your bag, or tic tacs or something like that so that whenever you feel like you might wanna cheat and grab one of them snacks you're trying to avoid, pop a piece of gum. chewing the gum simulates eating and it's been said helps you resist binge eating and eating when you're not really hungry.
if you do want to snack, why not bring something of your own in small portions, so you aren't tempted to sample everything else. there are a lot of healthy, tasty snacks out there.