How do I tactfully decline food that people buy for me?

I'm on a diet, but I don't really want anyone to know. Well, I guess it's not a diet. More like, I'm trying to eat as healthy as possible. I've started an exercise regimen and I stay away from junk food. Now, my boss and his wife have a habit of surprising me with fast food. Just today, my boss bought pizza for everyone. There were four slices left when I came in and they told me to eat it. I declined, but they kept insisting. I told them I ate before I came in, so they told me to eat the pizza later. This happens all the time. The other day, my bosses wife bought me french fries and a soda. I'm afraid I am offending them by not eating what they buy for me, but I'm doing so well with my diet! I don't want to tell them I'm on a diet because every time I tell people, they tell me I don't need to be on a diet and it just gets uncomfortable. Anyway, how can I politely decline the food they buy for me? The pizza is still sitting in the back room...