My friend is bothering me too much; how could I ask her to stop?

Theirs this girl I work with which we have become good friends. We help each other out at work, I do her many favors, and talk almost every day. But ever sins she asked me for my phone number I can't get through a whole day with out her ether calling me or sending me 20 text messages. She has a boyfriend and I have no interest in her. I don't mind being the male friend that she could tell anything to, but what really get me mad is that she thinks I want her. I say this because every time we talk and I mention some thing like going out with her or some thing of that sort she says no and calls me her buddy like to remind me that we're only friends. I have never done anything to make her thing that I'm interested, even though almost all the guys we work with have, she is not my type of girl, she's just a very nice person but that's all. she even knows I'm dating someone. It gets me mad that she would think that of me even after she tells me that she has a boyfriend and its just starting to bother me way to much. I don't know what to do, I don't want to lose her friendship. what could I do?