You want simple. Be her friend.
Dont have sex, dont pursue sex, dont flirt. Treat her like you would any other friend you dont have a sexual relationship with. When you meet up always do so in a public area so sex is not likely to come into play. If she asks to go some where more private bow out. Its important to Increase the time apart more.
If she pushes the issue tell her the truth of course. You care about her but you want to slow this down a bit because your not looking for a long term committed relationship and you dont want to lead her on. You can say it politely without telling her things you dont like about her. People dont change because you dont like something about them and there is no need to tell someone what you dont like about them. Its just hurtful and its more about you then her.
I know you dont want to hurt her but the longer you let it go on the more chance there is of that occurring. Your not responsible for your former “friend with benefits” poor decision in a current partner.
For future reference for almost every women there really is no such thing as FWB. We are emotion driven creatures and while we need a connection to a male to have sex with them to begin with sex always deepens that connection. Its the way we are made and how nature intended it. Those who can generally have some emotional disfunction or disconnect derived from past experiences. I am not saying men and women cant be friends. My best friend is male. Just dont have sex with them because that changes the dynamic. Its instinctual.
You will never make it into the friends box. By its very definition is means a male you wont have sex with but like. She already had sex with you. There is a connection. I know men are more instinctually driven but you need to know this before you have sex and be responsible for your part in it.
(((Joe))) I am shocked. LOL While on some levels this is true, you can have what you want without hurting others and still be considerate of there emotions and needs. The statement points something out though. Your speaking about someone you dont love or care about. He obviously cares although he is not in love. Honesty done correctly doesn’t have to be hurtful.
As for a pheromones that will put you in the friends box. Hmm thats a hard one. I would usually go with a women’s primary. I wish they had TAL still. IH can create intimacy if you use it but if you only use it when not in a intimate atmosphere around her you may be able to save the connection, have both of you open up about your true feelings and remain friendly.
She is sexually attracted to you and comfortable with you so heavy anone will just make her horny.
I am leaning toward P124, TAK or P85 if you can get your hands on those.
If not stick to IH and thats it.
Oh one more piece of advice. Ask your self what would you tell your son or daughter is they came to you with this problem. Ask it from both perspectives the male and the female one. You will know the right thing to do. After all you are someones son and she is someones daughter. 
Tisha