A boyfriend destroyer is basically constructing a
straw man and letting her tear him down. You can try to follow that strategy, or, alternatively, ignore the 'I have boyfriend' objection and treat it like a non-issue. This latter strategy entails that you continue to treat her as you have before you knew she had a boyfriend. I would think this is better in your case, because you would actually not
want a boyfriend destroyer to work on your potential girlfriend.
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One of the most useful things you will ever learn about
body language.