While I can't completely turn my ticklishness off, I find that sometimes when I'm tied down, it does get diminished. I think that there are two primary reasons for this -
First, when I know I'm about to be tickled, I brace myself for it. Which, I guess, has the tendency to lower the ticklish sensations. It's not something I can help, my body does it automatically. It's like I subconsciously want to give the 'ler a hard time and make him work for his giggles. I dunno.... I guess I love a challenge. So, if I happen to be with a 'ler who is only in it for the easy laughs, they might mistake my reaction as nonticklishness. But there is ONE way around this little defense mechanism - get me to laugh before you tickle me. Make it playful, be a total goofball, crack some jokes, etc. Tickmankyle1 - You might want to try this method on your stubborn 'lee. Chances are, if you've got her laughing hard first, she won't stand a chance at stopping the laughter after you start tickling her. Works like a charm for me anyway. It's like it bypasses my stubborn streak, and brings me straight into ticklish hysterics once the tickles begin.
Secondly, trust plays a HUGE role when being tied down and tickled. The more familar I am with the 'ler, and the more I trust him, the easier it is for me to be tickled into hysterics. Tickmankyle1 - how long have you known this girl? What kind of relationship do you have with her? Have you actually tried this scenario with her or was it just something you discussed? If you haven't actually tried to tie this girl down and tickle her, she could have told you about her loss of ticklishness because she either wants to put it out there as a challenge for you (In the past, I have denied my ticklishness a million times just to get a 'ler to prove me wrong), or it has been her experience up until this point that she isn't ticklish when tied down. It doesn't necessarily mean that it will ALWAYS be the case. Perhaps she's just had a string of really bad 'lers in her life.
If you DO get the chance to try this out with her, make sure you make her feel as comfortable as possible. Have her wear her favorite pajamas, play some upbeat music, keep the playful banter going while she's being tied down, don't get too serious your're first time around. Some 'lees get really self conscious when tied down, so try to minimize that as much as you can by telling her how great she looks. Start things out slow, light scribbles, kisses, nibbles, etc. Then when you feel like she's enjoying herself (you'll know by the expression on her face), take things up a notch. Again, make it fun.
Hope this helps,
Maggie