How to deal with people who use you?

I used to do favors for this girl out of companionship, even after I discovered that she has a boyfriend. But lately, I'm beginning to realize that she's just using me, because she just takes and does not give back. In fact, she will not even respond to my IMs, unless she needs my help on something the next day. What's the best way for me to handle this situation? Should I end all communications with her completely? (I still like her deeply, so this will not be easy) Should confront her about it, and maybe hope that she can change her behavior?