Trying to diet but parents keep eating my food!?

So i'm a college student that lives with his parents over the summer and I got EXTREMELY serious about getting in shape this past May. I've lost 26 pounds so far, I've been strict on my nutrition and going to the gym 5-6 times a week.

The problem:

Every Sunday I go grocery shopping for the week and prepare the meals im going to eat by putting them in storage containers for certain days (it helps me not stray away from my diet). Example: I have a big container titled "Monday" that has mini containers titled "meal 1, 2, 3..." My parents keep eating some of my food! I keep asking them to stop but every week something different is taken. It's irritating b/c i buy EXACTLY enough for the week, thus im put in one of two situations.

1) I find out early enough that something is missing and i can go to the store to replace it

2) I don't find out in time when it's time to eat and I have to run to grab something to eat elsewhere

It's starting to get out of hand, today with be my FOURTH trip to the store this week! For something I already bought earlier this week! The $$ for food and gas is adding up and i know tuition will need to be paid next month so I'm starting to get stressed out. What should I do?

I've even resorted to buying more plain-tasting foods like plain tuna and plain low/non-fat greek yogurt and it still goes missing.