Someone who interrupts you when you're talking to someone of the opposite gender whom you are interested in pursuing relationship wise and is not necessarily just trying to get in to her pants.
Their interruption usually ends the conversation you and the person of the opposite gender were having.
Tim: Yeah, the movie was pretty great. What's your take on it?
Jill: I thought it was good too. I'd watch it again.
Tim: That's good. A movie for home, then. So, I was wondering if you wanted to do something tomorrow?
Jill: Oh! I'd --
Steve: Hey you guys, what did you think of the movie?
Jill: Hey Steve. Tim and I were just talking about it, we both thought it was good.
Steve: That's cool. Well, I'm going to go get some coffee.
Jill: What were we talking about?
Tim: Oh, nothing.
Tim: Man, Steve is such a talkblocker.