Tart,
Have you tried asking her not to ?
If that doesn't work you could always try sending a few messages from HER e-mail address and "forgetting" to sign them with your name - If she still doesn't get the message, try picking up her car keys next time she leaves them on the desk and tell her you're going for a spin, or if you really want to strike a nerve try using some of her make up without asking
... Or ( If your e-mail account is only accessible when you are logged on to the Network, or to a particular terminal, ) you could set up a screensaver password to lock your computer when it's left unattended
