You have to be assertive but kind. I am one of those people who talks excessively so I know how to work it. Be honest. If you have to go say so and start to leave as you do.
If you just want to stop talking but you don't have an excuse you are going to have to be a bit rude. I do not suggest lying, but always make sure when you start a conversation with one of us, let us know ahead of time you have limited time.