Sad But True
Almost every woman has had to deal with a persistent guy at some point or another. They walk up to you and try to strike a conversation when you least expect it and they just won’t take no for an answer. Even if you’re honest and tell them that you’re not interested, it still doesn’t seem to work.
Infuriatingly, the only thing that does work is to tell them that you have a boyfriend. As this OP pointed out, it’s sad that that’s the only way to get them to back off. Basically, they’ll respect a man they don’t know before they respect the woman in front of them. Isn’t that just awful?