In reality the only person we have complete control over is ourself. All you can do is ask someone to stop their behavior. If they don't you always have the option to not to be around them. Each of us chooses our own friends, lovers, and spouse. Ultimately everyone is looking for someone who will love and appreciate them for who they are. Some people see flirting as being "harmless fun" and others view it as being "testing the waters" to see if there is any potential chance of getting together.
Whenever someone takes offense the person flirting falls back on the old line "I can't believe you thought I was serious!" or "Stop being so jealous and paranoid!"
Remember you are entitled to have own likes, dislikes, and "deal breakers". You are responsible for YOUR own happiness. You don't have to "deal" with anyone you don't want to.