This might be an obvious question - but... Are you putting your name on them? in big letters. Maybe try some other decoration. If your yogurt is less obvious to conceal, people might not take it if they can't eat it without suspicion. I think the sabotage idea is funny! I would suggest a laxative, but that would just be mean.

That kind of reminds me of the video I saw on TV once of a man peeing in the coffee pot in the break room because he was mad at his co-workers. Ewww..