It is hard to think of polite ways to say this. If you tell someone they are talking too much, they may become offended but sometimes you just have to let them know. You might instead just excuse yourself by saying your schedule demands that you be elsewhere.
Another approach is to say something like "Thanks, I understand what you are saying. There is no need to explain further."
Another approach is to interrupt and talk their ear off, thereby giving them a taste of their own medicine!