[Xgenre]

The most respectful way is to be honest. Tell her you wish to stop and that you will like to remain in touch. Given that you have seen and done all with one another, there will be an unusually intimate level of honesty in the friendship. Nothing to hide, no need to be politically correct.

Guys only need lust to have sex but the girl may have some feelings for you which is why she agreed to be a FB. No need to do or say hurtful things in the hope she distances herself. You will just make her feel like she is a piece of used tissue that you are trying to throw away after you had have your fun.