Sometimes you can see that the person you are talking to is really scanning the room for someone more interesting/available/richer or prettier to talk to. Let them. Just smile broadly and excuse yourself.

Remember, your house, your rules.

Be brave and give them a taste of their own medecine - most people are oblivious of their behaviour and will generally respond quite well if you point it out to them. How you do it depends on the situation and people; for pushy, loud people - be direct, they can take it. For snippy, bitchy types just be devastatingly nice in your delivery and smother them with loveliness - it will catch them completely off guard. But try to be polite when you do it. Most of the time, obnoxious people turn out to be much easier to deal with once you have broken the ice and made them aware of how much they irritate you. And if they don't then at least you have an understanding and can avoid each other next time round.

If they are being truly obnoxious, simply go over to them and quietly tell them that they are being too disruptive for this gathering so would they do the stand-up thing and please leave. If you are really nervous about doing such a thing, ask a close friend to back you up while doing this. Do this discretely as possible so as to not disrupt the party. As the host, you should ensure that your guests feel welcome and good about being at your party. So don't have a second thought about booting someone who is spoiling it.