observe her while she cleans your room, tell her what things are to be kept and what are to be thrown away, or ask her to use a separate box you can pick through later for things that aren't obvious trash(like wrappers or bottle caps or what-have-you).
or maybe keep your own room clean, no need to be an starfish about it. she was only trying to help, and instead of being nice about it, you decide to take it offensively.