Firstly, sorry to hear that you're in this situation. Theft obviously isn't legal, but when you share common areas with people, there's no real way of prosecuting them for taking some food. How do the other housemates feel about this? Honestly, the only thing I can think of is sort of talking to them and escalating it to your landlord about what she's been up to. Threatening to leave will help as they would much rather eject one person out of the house than having four leave. Otherwise, really the only thing I can think of is moving out and finding alternative accommodation. If you're tied down to a contract you might be able to waver it because of how you're bring treated. And surely illegal activity, like those parties, should hold some value in getting her removed. Make sure you get evidence when she does things like that.