Depends who it is and how well I know her.
In 8th grade a girl I had never met before repeatedly hit on me at lunch. The very minute I met her she asked for my phone number. I didn't know her, I didn't find her attractive, and I didn't like it that she was asking for my number when she didn't even know my name or anything about me yet. So that time I really disliked it and told her bad things about me in order to make her stop, but none seemed to work. Then, after a few days I told her I didn't believe in god, she looked shocked and stopped bothering me from that day forth. It was a huge relief.
Also, if it's someone who I myself have feelings for and I know that they don't like me like that, it really irks me when they compliment my appearance because it always comes off as flirting to me. I'm not sure if that counts as flirting. But say I'm trying to get over some girl, but am still trying to be friends with her, and one day she's rather complimentary--it really mucks with my mind because I interpret it as such, and it just adds uncertainty and, despite the fact that they're compliments, worsens the insecurity that inevitably comes with crushes. I can be vocal about this or just bury it inside, but if we're communicating through text then it's more likely to be the former.
However, if it's someone I'm at least a little familiar with and I reasonably like what I know of her personality, and I find them to be at least a little physically attractive, then I may just kinda smile or I might flirt back slightly. Usually my flirts are very subtle however--small stuff. Glances or expressions or compliments, certain tones of voice, etc. However any flirtation tends to be rare.