Don't get naked with them. (from your other thread, that was clearly an issue!)

Don't give random hugs or physical affection (like... if there was a total breakdown then sure, but don't offer it really, and decline it when they initiate).

Don't hang out in your bedrooms, or in your private living spaces where no one else would be.

If they talk about something that is inappropriate, remind them that you are only a friend and don't want to keep hearing about how they care about you as more or want to have a physical relationship - make the boundary really clear.