I couldn't find a sticky note (I either misplaced them or they got stolen, you've no idea how organized my room is despite the apparent mess), so I took a piece of one of the notecards I'd cut into fourths to study Latin with to leave this on the bathroom mirror:
All of this passive-aggressive stuff is ridiculous and needs to stop. If you've got something to say to me, say it to my face. I don't care if it's one or the both of you, she's your guest and you're still responsible. This is the last little note I'm leaving and I won't be reading anymore of yours.
That should go over smoothly. (Between the lines: Grow up.)
Should she leave another note, provided she's not watching me I'll read it out of simple curiosity, but then I'll just throw it away. If she is watching it goes straight in the trash.
In the highly likely event that this solves nothing, I'll play a mirror game until I leave. (I.e. You move my washclothes I move yours in the exact same manner, you throw a hissy fit and throw my things around I'll do the same to yours, you get the picture.)