The longest way is, the next time she asks tell her, "Oh! I'm sorry, but I can't afford to buy us both lunch, I only have enough money to buy for myself today. Maybe another day, huh?" And, next time, tell her the same thing but in a different way. Make sure that you put just enough money in your wallet for your own lunch. If you use a credit card, she can't know you aren't telling the truth. That's the hard long way to do it.
The easiest way is to just tell her, "I can't afford to be buying you lunch everyday. I like eating lunch with you but only if we can pay for our own food and drink from now on." If she takes offense and walks away or says never mind then, make a sad face and tell her, "I'm sorry sweetie, but thanks for asking."
You are an adult and you know she is doing this on purpose..so there is no need to be nice. Niceness should only be given to people who deserve it. You tried to be nice a couple of times or so already, but that's enough. If you keep it up, it's not shame on her anymore, it's shame on yourself for letting her take advantage of you. Good luck, cuz these awkward situations are always pains in the butt! ;)