What do i do about know-it-all at work?

I am a graduate student and an employee at an on-campus organization where I am a student writer. I am also the editor of a newsletter that we publish. However, there is a know-it-all full-time employee who keeps trying to into the newsletter by trying to edit it (although she isn't even a staff member). Today, while I was teaching a cl, she went to my desk and took a copy of the newsletter (the unedited version, I might add) and flipped out on a bunch of people in the office (also making a bunch of edits). She is ridiculous and should not have been taking things from my desk. She is not the editor anyway. What should I do about her? I don't want to be on her bad side since I basically share a workspace with her, but this is ridiculous.