In cases where you have to tell someone things that they probably don't want to hear, these are the principles that I generally try to abide by, and they have served me well.
- Be clear but not unkind. Try not to leave the door open for her to misinterpret the situation.
- Avoid lying or making excessive or elaborate excuses.
- Remember that you are allowed to set and enforce your boundaries!
- You are not required to to justify or explain why you don't want sex (although you can give your reasons if you want to).
- Don't try to 'soften the blow' or avoid conflict by allowing her to think it might happen between you when you know that is not what you want.
- Let her know your expectations in advance, so she knows she is agreeing to a platonic visit only before she solidifies her plans.
In this case you might want use words like, "I'm happy to have you visit as a friend" and "My guestroom is always open to you." You may also decide to tell her that while you would greatly enjoy her company, you're seeing someone else now. She does not need to know the details or exact nature of your new relationship unless you wish to tell her-- honesty without oversharing is the goal here.
Avoid getting into situations that send mixed signals. Don't do things that appear to be leading to a possible romantic encounter, like drinking heavily together, cuddling on the couch, etc. If you want to be just friends, keep things on a friendly level and enforce your boundaries firmly but kindly as needed.