You said you've already broken up? You did the "I'm breaking up with you", "This is over", "I'm sorry, we're not going to be together anymore". Not "I'm gonna go stay at my parents for awhile"... etc and make that *clear*?
You should do it face to face, and in private (not in a restaurant or a park). You should have an exit plan. After you're done talking, go for a walk, out for a coffee (alone you fool), or just to a friends place.
Be direct. Use "I" in sentences, not "we", and never "you". You want to communicate that there ARE reasons you don't want her to be there. But you don't want to be lecturing on what they are.
"Now that we're no longer together, "I" want you to leave". Something like that
If she gets the message she'll
1. leave
2. realize that she doesn't want to leave, maybe start having sex again to stay.
Be clear and stand your ground. Stay strong.