Originally Posted by essxjay

...Another reply to the FA might be: "I'd like to help out as long as I can be accommodated in another aisle seat. What specific seat did you have in mind?" If FA doesn't know offhand or simply presses harder for a unguaranteed favor then calmly inform them "I'm claustrophobic and cannot tolerate middle or window seats. I'm sorry, but my answer is 'no'." It's polite but final and should be perfectly acceptable without further discussion.