Why Are You Still Talking To Me

I have always thought that the way people show interest in what someone else is saying was universal. You respond to what someone is saying and/or ask questions. You become visibly engaged in what this person is saying. You interact. I like to think of myself as the type of person who knows when to shut up in response to the lack of interest I feel coming from the other side of a 'conversation' and I like to think that other people would know when to do the same.
It would seem that this is not the case.
Being the stony, anti-social loner I believe myself to be, I feel that I have mastered my uninterested face as well as my steely 'Why the hell are you even looking at me?' glare. While I do believe that no one is immune to the silencing effects of them (usually paired with the simple act of walking away), some are just stupidly oblivious.
How is it that someone can be so self-involved to continue talking to someone, WITH ENTHUSIASM, who is so clearly not giving a damn about what you are talking about?
As my current beef is in the workplace, I can't pull the stunt usually reserved for strangers: Either walking away from someone I don't care about at the pivotal moment in the middle of their sentence, or stating, "I didn't come here to talk to you" and then walking away. My goal is to leave the other party with a sense of hollow abandonment.
To deal with this particular person, I've developed a 'staggered response and ignore' tactic which seems to be breaking them down. As this person is known to emotionally purge on everyone they come into contact with, I can't simply cut them cold turkey. I don't want any tearful, lengthy emails sent to me on an hourly basis and I really don't want to be named in anyone's open suicide poem to the world. That would just make me look bad for not being a coddling enabler.