I think locking up the pills or simply removing them from the house if nobody uses them is the right first solution. Sometimes that may be all that's necessary.
When I was a kid my stepmother severely rationed my food (it was a control issue, we were not poor and I was far from fat, I was a skinny beanpole), and I was ravenously hungry all the time. I did a lot of yard and house maintenance work for various neighbors, and occasionally while they were out I would nip into the kitchen and steal a few cookies or slices of bread. They noticed, though, and put the food elsewhere so I got the message. They never said anything about it to me, nor I to them, but we all knew and I learned my lesson. That worked out fine.