Ahhh that sucks!!!! My niece used to steal everything!!!! Had she just ASKED, I would have let her use whatever she wanted. She also lied about it too, which pissed me off to the high heavens. I know how you feel & its the worst...try to remember this situation is temporary. She grew out of it with age, but I would have to hide stuff & search her room
Locking your room is a start. Also, try gently (or not so gently) reminding her that her behavior is a very disrespectful annoyance you don't need. It doesn't matter if she was stealing lawn clippings. She's lucky you're not threatening to kick her out!