You can get a lock box to put over an outlet for under $20, maybe even less. You just need to get one big enough to hold the plug-in as well.
These are most commonly used for one of two purposes: to lock outdoor outlets, or to lock thermostats. But they’re pretty easy to find.
Home Depot should have several, or you can go on Amazon. Here are just a couple of examples.
The following would be a bad idea, because it’s a metal box and it will be hard to get signal from the smart device out of the box. But something in this shape made in clear plastic would probably work well.
Also try to avoid blue plastic, for whatever reason it often seems to be harder to get signal out of than clear or beige or even smoke colored. .
If you get a Dual gang lockbox ( search term: “lockable outlet cover“), there should be plenty of room for the smart device:
Which ever locking cover you choose needs to be combined with a smart control device like a plug-in pocket socket so that you can put it on a schedule or turn it on and off as desired.
There are some pretty small pocket sockets now, so you should be able to find something that will fit.