When there are no words left to say. If it has gotten to the point that there isn't anything left to say. You've exhausted all of your communication skills. You've spoken, yelled, screamed, complained, pleaded, begged, etc. until you're blue in the face. You've made several attempts to repair the situation or the relationship, only then are there no words necessary. Except good-bye. Only good-bye or saying nothing at all doesn't allow closure in a relationship. You may have said all that needed to be said, however, your partner may not have. The question then becomes, is it fair to simply walk away with no words because you feel like you've ran out of words.... now it's time to listen!
If the other party isn't speaking then you've done all you can do, but give them the opportunity to speak.
Prime example, I find in many men. It's been going on for ages. When a man feels like they've out-stayed, out-grown, or grown-weary of a relationship, they use silence as they're way to escape. When you shut off the communication device, it leaves room for anxiety. The woman is now questioning herself, her actions, what did she do, where did she go wrong? The man hasn't given her the opportunity to close the chapter in her life. He's probably said a million times, "I wish you'd do this." or Hinted around in some way the things that are steering him toward the decision to leave only we didn't hear him, or if we did, everything he has said went right out the door when the day comes that he walks away without words.
That's why we see so many dramatic situations when it comes to break-ups in a relationship. That's why women react irrationally when they don't hear from the man. Because we haven't been listening. That's why it's sometimes important to say it one more time, to express in words the reason why you're tired of preaching, so to speak. This allows some closure. It forces the other to self reflect even if for a moment, a second, etc. It alleviates unwanted drama. It allows both individuals to know what the other is feeling and the reason behind their actions.

So what do you say when there are no words?

You say, "I've exhausted myself trying to repair things (whatever it is). I don't want to do it anymore. I could go into detail about it, but all I can really say is look back and think about it. Then you'll see the whys. There isn't much else to say but................etc."

To me there is always time and a necessity for a final word, if for no other reason, the closure.