How we change what others think, feel, believe and do
Introduce a friend
There are a number of ways to use the 'introduce a friend' method to close a conversation. When the new person is engaged, you can then excuse yourself and move on.
Grab any person nearby, asking them what they think about something. Explain how you can't decide on something. Most people are happy to offer an opinion, even to a stranger.
As you go around the room talking with people, remember what each is interested in. Then when you want to back out of a conversation, draw in a person you already know has something to say on the topic. This is easier if you have done a quick circuit of the room first, just listening in to various groups.
Take somebody with you when you talk with others. Say 'let's go and talk with that person'.
Jim, what do you think about this?
Let's go and talk with Celia over there. She's very interested in these things.
Sorry to trouble you but we're rather stuck here. Do you happen to know the name of the President of France?
Bringing a third person into a conversation does not necessarily end it, but it does add variation. If you want to change the topic, it is easier to do it through a third person, particularly if you get them talking about something in which they are interested.
A third person also provides a good excuse to move on as you are not abandoning the other person to stand alone, which is generally not considered very good manners.