So, this has been a topic of some thought and discussion in my friend circle as of late, and unfortunately there isn't any one good general answer to it.
If you're not actively flirting with someone (and even that is an altogether too-broad term that encompasses an extreme range of various actions), there's not really much you can do to avoid giving that impression. If someone is interested in you, they will generally be perfectly capable of interpreting anything and everything you do as a potential sign of interest. Especially if they don't know you particularly well.
Approaching the subject after you've identified it is also, well, somewhat awkward... If taking the direct route and just telling the person that you've noticed certain trends and that you're not interested in that way, it's absolutely not uncommon for them to deny everything and say that you've got it all wrong.
And no, trying to put the brakes on something by turning on the "freeze factor" and attempting to give signs of disinterest really doesn't work. Not only does it mean being unnaturally distant from someone you otherwise get along with, but a number of people are extremely attracted to that kind of aloof and cold behavior.
There are optimal scenarios where this situation can be neutralized with minimal hurt and disappointment, but those scenarios cannot be guaranteed. You really just have to respect her feelings as a person and tell her, presuming that there's absolutely no chance that you'd be willing to explore things with her.