<human>: Can you summarize the given text in a few sentences?

I'm 17, and my girlfriend is 15 and we've been in a relationship for 7 months. She's been sheltered for a lot of her life by religion and doesn't recognize it when people flirt with her-- she just thinks they're being friendly. So her response is to be friendly back, which sometimes seems like she's flirting. It's been something we've been working on and she's getting a lot better at turning people away but there's still some times when I'll see that she's been facebook messaging a guy from school about ASB or something and he's really trying to flirt with her. She does all the things we've talked about to turn him away but he's still persisting. I've messaged the guy letting him know that we're happy together and that I can see what he's saying, but he's still going for it. Is there anything I can do to get guys like him to stop flirting with my girlfriend, knowing she'll just respond back not knowing what their intentions are? I always feel like the bad guy when I have to tell her to stop talking to someone that she thinks is her friend because I "think" he's flirting with her.
<bot>: How do I keep guys from flirting with my girlfriend, and explain to her when someone is flirting and how to respond?