This happens to me too, all the time. My strategy which worked for me every time: take a slice and package it up for later.
I realized I wanted the pizza or other free food because a) I wanted to join in on the fun my coworkers were having b) I wanted to take advantage of free things and c) I loved the taste of pizza.
However, I knew that if I indulged in what I wanted at that moment, I wouldn't be able to stop. So I would keep a Tupperware at work and take one slice (or even two) with me and keep it in the fridge. That way I didn't feel like I missed out and I was able to enjoy the benefits of free food. My "craving" to be a part of the free-food fun was satiated. I didn't have the pressure to make a decision about eating as much as I could right now because the pizza (or dessert, etc) would be gone. I had a small reserve that belonged to me that I could access at any point, so I didn't have to go in to the conference room and gobble up as much as I could stuff in my mouth before it was gone.
I could then make a RATIONAL decision later on (at dinner, etc) about how to incorporate it smartly into my diet rather than an irrational decision immediately at he time and usually wind up messing up my whole day. Having that "reserve" allowed me mull over whether or not I actually wanted it/needed it right now, and usually convince myself I didn't.
