Quick little tactic that absolutely wrecks anyone trying to butt into your conversation with a girl.
Every guy's been talking to a girl only to have some random dude rudely interrupt his conversation, either trying to talk to him or trying to talk to the girl.
Sometimes it's because the interloper is uncalibrated and just wanted to talk to you or her but did not know how to wait properly for an opening in the conversation to jump in.
Other times it may be because the interloper directly wants to steal your girl, and he's hoping to peel her off you, or to peel you offer her (either so his wingman can talk to her, or so he can back-turn you once he's gotten you to break circle and turn his attention to the girl, with you now out of the conversation).
My normal recommendation (and normal policy) is to just ignore the guy (see: Dealing with Disruptive Men).
Most guys won't be able to break into a conversation if you don't acknowledge them, especially if the girl is into you enough to follow your lead and ignore the guy so long as you're ignoring him too.
But what do you do if the guy is really loud, aggressive, and in your face?
What do you do if he approaches the girl first, and you can see she's about to crack and break circle to engage with him?
There's an alternate tactic you can use -- something of a disruption Plan B -- if you're quick enough on your feet.
I call it the 'Disruptor Destroyer'.