There's a couple things you can do:
1) Be blunt with her and tell her that you should stop seeing each other. OR
2) Leave things up in the air.
I ALWAYS prefer to leave things up in the air for a few reasons:
From the sound of it, you and her are not official, so you don't really need to "break up" with her. It seems like you guys hooked up a few times but never took it further. This to me does not warrant "the talk."
If you are blunt with her, it might make things a little awkward since you were never really official with her. Also, why not sarge other girls with her around? If you and her were never really official, your conscience should not even come into play. Why define something that hasn't been defined? It takes the fun out of it.
When she knows that you're not as attracted to her as you were before, she will want you even more. And who knows what can happen in a month or even a year from now. You don't want to shoot yourself in the foot by having "the talk" when you don't need to.
Another word of advice: to prevent your conscience from ever coming into play, you need to be blunt with her BEFORE hooking up with her. Most guys think that this will prevent them from hooking up in the first place, but it doesn't. The key is timing. In the heat of the moment (right before you hook up), if you tell her that you're not looking for anything serious and she still hooks up with you, it's her fault. You did your job by giving her a disclaimer. 9/10 she'll still hook up with you anyway. This will prevent her from spreading nasty rumors about you and tarnishing your reputation.
Hope this helps!