I'm not going to read this whole thread, so pardon me if someone has already suggested this.
I attended college/lived in a dorm in another country where the cafeteria was closed from Friday afternoon until Monday morning. The one grocery store was not that close and it closed early too. Fast food places were nonexistent. I didn't have a car....few of us did and the buses ran infrequently on weekends. In other words, getting food over the weekend was difficult. They didn't even sell much in the way of microwave meals, but if they had, it wouldn't have helped, because someone would have stolen your Lean-Q out of the group freezer and eaten it.

(We couldn't have individual fridges.) Buy lunchmeat, etc. and the same thing happened....someone would steal it, labeled or not. I lived on cup-a-soup most weekends because I could keep that in my room. One weekend, I bought this stuff called breakfast yogurt that had granola on top. Yep, it was stolen.
A dorm veteran suggested we make that yogurt a little less appealing to the thief next time. What a friend she was.

I bought more the next weekend and we loaded it with powdered laxative. The thief swiped it, gulped it down, and presumably spent a miserable weekend in the toilet.

And the fridge theft slowed down considerably after that.
So.....Why not make dear old Dad's favorite flavor of pudding, add something special......not too much.....let it sit on the counter and see what develops? Maybe if your pudding doesn't agree with his tummy, he'll leave it alone from now on.

Suggest to him that he may be lactose intolerant.
I don't care if he IS her parent, scarfing down 100% of the pudding that you KNOW someone has just made is plain old rude.
But it's JUST a suggestion..............