You know you see them.
There are other people waiting to talk to him or her. Waiting and poking their heads around that glass wall you've just put up as you continue to pitch your project, but what happens next is up to you.
Do you hold off as long as you can?
Do you duck away and let someone else in?
Do you turn to them and incorporate them into the conversation?
I would argue that by making the choice to include someone else, you have accomplished a number of things. You, first and foremost, are making sure that the other person gets a chance to talk to this person, you are also providing more to talk about because now it's not only about you and your project, and, finally, you have put yourself in a position of power in the conversation because you opened it up. You didn't fight to keep this person out, you let them in and they'll remember that.
Now of course you'll occasionally encounter some who will try and monopolize the conversation and push you out but by then you're probably on to bigger and better things. Remember that a good networking conversation should be succinct. You should get to know them and let them get to know you and then move along. If you can introduce someone else in the bargain, then you look even better.
Be careful of closing the door on others; you never know how they may react in the future. And nobody wants to end up in this situation.