Its okay to establish boundaries. Tell her you're too busy to reply. You don't need to explain yourself. Just be nice, but not too nice. Keep your distance. Eventually she should get the hint. Or you can actually tell her, "I gave out my number just for the project. It costs me money when I get texts so only text or call me if its an emergency." I had a friend who would text me upwards of 10 times a day, every day. I eventually had to lie and tell her she was overcharging my cell phone bill because each text costs money, and she should only text me if it was absolutely necessary. That was the only way she stopped.