Girl takes my friend's claimed seats at lunch?

At lunchtime, I have this "friend" who thinks we are like best friends and wants to sit with me like at ALL costs. I try my best to sit in an empty chair between two girls that have already claimed their seat visibly(with a bag or food plate). That used to keep this "friend from being able to sit by me. Now though, she just doesn't care if there is a bag or anything in a chair next to mine, she just moves their stuff to another chair, even when they are coming up to the table...They will usually confront her about it and ask why she moved their stuff, but she just says "oh well it will work." And doesn't seem to care or realize she is being really rude. I've even tried telling her nicely when I see her moving people's stuff around that they probably don't like it when she does that etc. She just isn't getting the hint. How do I and my other friends tell her in a tactful way to stop? We are seniors in high school.