That is hard. Does the court buy or do people take turns? If the court buys, talk to the buyer and request a salad with the pizza. Or order it when it's your turn. Chances are, other people will appreciate it and the trend will continue. Or just bring a salad when you know it's a pizza day. Enjoy your slice and fill-up on the salad. Sure, it's not quite the free lunch, but it's cheaper than a new wardrobe.
Another idea is to get the whole department on your side. Rope in HR and get a weight loss team going within your department or even company-wide. The company benefits because healthy employees cost less to insure. Tie in with Biggest Loser, or WW or something. Then you'll have more support in trying to change the treats or how the group celebrate things. What about flowers instead of food?
Back in the day, I used to give up sweets for a month when I needed to get my weight back on track. [sigh--those days are long gone] I'd allow 2 cheats, often planned for the birthday celebration or when I knew there was something good coming up. Other times, I'd ask myself it this was worth breaking my promise. Usually it wasn't. But if it was, I knew I'd enjoy it and I'd recognize it as a treat vs the norm.
If all else fails, make it count. Nutritionally, that is. To fill yourself up until the next meal, you need a meal/snack with sugar, carbs, protein and fat. The sugar kicks in first. As the energy from the sugar is dropping, the carbs are giving you energy, then the protein, and finally the fat. (Or those last two could be reversed, not going to check right now) Anyway, if you augment the treat of the moment to be more balanced, you can carry yourself to the next meal with little consequence. So say there's donuts at 10. Have one and add a handful of peanuts and a tub of applesauce (both desk-stable) and that should carry you til lunch when all you'll need is a small chicken salad. Otherwise, you eat the donut, maybe 2, crash by 11 and eat an extra large lunch because you're hungry and tired.And maybe get stuck on a sugar/fat cycle where you're sluggish and over-eating all day.
Best of luck!