What's the best way to deal with a person that rants too much?

I would like some actual words to say, even if it's a script. There has got to be something to say when someone is going on and on about what angers him/her, whether it be a story on the news, the tv remote not working, the dog getting sick on the carpet. This person acts as if he has a right to express his feelings, which he does. However, doesn't it cross a line somewhere when the rants get so heated or lengthy. It seems so destructive. Any words someone can suggest to me would be greatly appreciated. This person also looks for opportunities to supposedly avenge wrongs but I get the sense it's not a cause that he would normally avenge. He's just using that as an excuse to express anger. Even if these causes were things he felt passionate about, that does not make right his way of dealing with it. All these things have been said to this person before and I'm not looking for advice such as, avoid the person. I want advice in the form of actual words to say to him or to our doctor. I already asked the doctor to ask the person to pay attention to the energy he gives out and the doctor acted like he can't help it because of his head injury. I don't know if this person was like this before the injury. Something has to be said. If anybody knows of anything said that actually does somehow penetrate, make a difference, make them think, make them seek to get help to be more constructive, I'm all ears. Thank you.