I wouldn't do what MLZC suggests, myself (I have an 11yo dd1). It might work for younger children, but I think this requires a more thought-out response.
I'd make it clear, very emphatically and seriously, that taking money (or even going into your handbag, actually) was totally unacceptable, for whatever reason (joke, dare, needing cash etc), and how serious it would be if you felt you couldn't trust her.
Then, very calmly, I would try to find out the reason she was doing this, so that you can discuss alternative scenarios. If it was a dare, you need to have a chat about the need to make good decisions when other people encourage her to do silly things. If she needed the money, or wanted the money to buy sweets or whatever, then there needs to be a serious discussion about how much pocket money she gets, what she is expected to buy with it and how long it should last.
In the end you want to guide her towards a mature attitude where if she genuinely feels she doesn't have enough money, she could come to you and put her case, while also realising that your property is yours alone and that she has no right to tamper with your belongings.