Originally Posted by R600DuraAce
Up ahead, I saw this dude on a MTB with slick tire and SPD pedals. I just rode past him. 50 sec or so later, he was coming up on my left side. Then, he rode ahead of me. He slowed down again. And I passed him again. He did the same thing. Rode past me on my left and got to the front and slowed down again. This went on for 3 or 4 times. I got very annoyed.

I can understand your irritation. On one commuting route I had a similar fool. What's aggravating is that they slow down after passing, so that you can't even keep your normal pace and you are forced into a little passing game if you want to get to work on time.
You did right in just blowing him off for the day so that he couldn't pass you again. But if you want to avoid it in the future, I suggest finding an alternate route when you see him. People like that are as weak in the mind as they are in the legs. If they see you, they will never leave you alone. That's what I did, just found another longer route where I didn't have to over-exert myself. Good luck in the future.