How to warn my roommate not to tamper with my things?

You can directly talk about your personal habits. You don't like people touching your personal belongings. If you want to use them, you can ask for them. Don't resolutely say that you can't do anything, otherwise it will be difficult for everyone to live together. If you say it, everyone will pay attention to it. If you don't say it, it will be their own use. Speaking in advance and speaking afterwards gives people a different feeling. Speaking in advance avoids embarrassment and everyone is easy to get along with.