1. Can you cut off you're ticklishness?
Yes, I have sometimes been able to do this, but it is not an easy, flip a switch kind of thing for me. You sort of have to psyche yourself up for it. Oh, I should also mention, that if you really want to be tickled at that moment in time, it prolly won't work.
2. If so, How the HELL do you do it?
It's a mind trick, purely concentration. Your mind overrides the feedback from the body part being tickled. Have you ever carried something so cold in your hands that it actually hurt to hang on to it, but you knew you still had to carry it another 50 feet before you put it down-- so you forced yourself to ignore the pain and discomfort of the cold object? That's the closest, non-tickling analogy I can think of.
3. How did you learn to do it?
well, you could try having someone tie you down and ruthlessly tickle torture you as you try practicing what I said above. That's pretty much how I learned how to do it, I guess.
And, by the way, here's the question you should have asked:
#4. How can I break someone who is doing this?
I make you no promises, because I can only tell you what has worked in my personal experiences.
For me, the whole thing relies upon my ability to concentrate. I am assuming, perhaps incorrectly, it is the same for other ticklees. Surprise them, break their concentration and everything "resets." Tell them one dirty joke after another, blindfold them and do hit and run tickling until they crack, gang up on them with multiple ticklers, pinch their nose and make honking noises while they get tickled, if they have another kink/fetish then use that to distract them. I think you get the basic idea.
The catch is that the sensations tend to build more as I resist the response. So, while I'm giving it to the LER, I'm getting all wound up inside at the same time. lol Ahhh, such a charmed life! 
