Avoiding "Cheaters" in Office Weight Loss Competition

If the goal is to encourage weight loss and have goals/prizes here is what the group uses that I am in that is through a running store but you could adapt to your group. Like we can earn back the whole $50 if we meet basic goals in gift certificates but you might want to use smaller one s to allow for the "other" milestone incentives. At 2lbs per week after 12 weeks you are looking at about 24-26 lbs lost healthy.
We pay $50 for a 12 week challenge.
Receive $25 Gift Certificate once you complete 8 weigh ins and lose 2% of body weight ( 200lb person would need to lose 4 lbs) you could use $5 gift cards to nearby shops, food places, coffee shops, etc


Receive $25 Gift Certificate once you complete 12 weigh ins and lose 5% of body weight (200lb person has to lose 10 pounds)
We also have small prizes based on Race lengths but there might be some numbers that mean something to your group

5K Club(3.1 lbs lost) - a prize
10K Club (6.2 lbs lost ) - a prize this one was a small water bottle
Half Marathon (13.1 lbs lost) a prize
Marathon Club (26.2 lbs of course) a prize
Ultra Marathon Club (26.3 lbs lost) a prize and a well-earned pat on the back

You could set aside a smaller amount ($50) to use for a leader prize but nothing too big. This way every one can get something back and be rewarded with something besides better health. It might also discourage

Nothing was very expensive, water bottles(light plastic ones). stickers, towels, headbands, whistles, etc. but always fun to get something. These prizes were not announced ahead of time either. And I like that the certificates are tied to actively participating each week or completing weekly check ins. Your could also check with your company's HR or Marketing departments for leftover giveaways as possible prizes.

Hope this helps or gives you some ideas on how to something to the one you have.