The main key is don't let them take over the frame. If another guy comes up to a girl while you're talking to her, say what's up, ask him if he needs anything, and then ask the girl if they know each other. If she says no, politely tell the other guy that you're having a conversation and then physically block him out.
I don't really see this changing a lot based on venue except that in a place like a bookstore it's much more likely that the guy actually does need something or know the chick as opposed to trying to pull a robbery on you (that would be really hilarious to see actually).
I don't really see this changing a lot based on venue except that in a place like a bookstore it's much more likely that the guy actually does need something or know the chick as opposed to trying to pull a robbery on you (that would be really hilarious to see actually).