"Dear Aunt Traci,
Much to the annoyance of my kids, I am a very friendly person. Wherever I am, I talk to people. Cashiers, old people, dogs... I can find something nice to say to anyone. "That sweater looks great on you!" "You like Dolly Parton? So do I!" The problem comes when people mistake my friendliness as an invitation to ask me out. I'm not interested in that--I'm just being friendly. How can I prevent people from thinking I'm a big flirt?
Well, I guess my answer depends on the kinds of things you say. There are some things that are obviously provocative, such as "Your butt looks phenomenal in those jeans." Even if you're saying it in a completely harmless way, there are some people who will mistake that for hitting on them.
Here's my advice. Either you change what you do so that no one gets the wrong idea, you continue being friendly and when someone thinks you're flirting with them, correct their misperception, or be more aware of the context and how you're being seen. For me, the answer was the third one.
It all comes down to self-awareness. I used to have a problem where I thought I was invisible. Literally! I had no idea that other people even paid attention to me, and so when someone said "Hi" I acted like a deer in the headlights." Awhile back, I was getting coffee at 7-11 and a man said, "Hi neighbor." Turns out he is my NEXT DOOR NEIGHBOR and I didn't recognize him. So, self awareness is an area of growth for me, and it might be for you too.
I'm still as friendly as I ever was, but I've installed a slight filter for what comes out of my mouth. It goes like this, "Is what I am about to say going to be taken that I am hitting on the person? If so, do I really want to say it?" I've removed my invisibility cloak, so if you see me getting coffee and say hi, I'll probably say something back.