In my experience, if someone is that focused on how you did a trick, it's because they feel the point of the performance was to figure out your method. In other words, you presented it as a puzzle, a challenge. You fooled him, and the unspoken deal is you show him the solution to the puzzle.
Look at how you perform. Since you called "David Blaine's Sandwich Trick" I'm going to make the bold guess that you both learned this from YouTube, and also that you used Blaine's style of minimal scripting without understanding why that style of presentation works for him. Doing that generally means there's nothing for the audience to connect with other than figuring out the method.
There's also a big difference between, "How did you do that!" and "How did you do that?" The first one is just an exclamation of surprise and wonder. That's not a bad thing. The second one is a focus on method, and that is bad in my opinion.
If someone is pestering you for methods, politely tell them to leave you alone. "Hey man, I'm glad you're excited for the trick but I'm not going to tell you how I did and you're disrupting these folks' fun by following me. Please stop." A lot of people just don't realize when they are being rude and if you let them know, politely, they will correct their behavior.
Another thing to do is use the crowd as a way to control an unruly audience member. "Hey folks, it's been fun performing for you, but we've got someone here who is just dead set on trying to get me to tell him how I do things so I'm going to have to pack it up and see ya'll later." If the crowd is enjoying what you're doing, they'll shut him up for you.
When people ask me to do a routine again my reaction depends on what routine it was. Much of what I do is repeatable, so if it's appropriate I will just do it again. But if it's a routine that shouldn't be repeated for some reason, my easiest way to get around their request is to say, "Sure! C'mere, check this out" and then I'll do whatever I feel like doing. If the first person says they want to see that specific routine again, then they're probably just trying to figure out the secret. In that instance I'd say something like, "It's not as fun if you already know how it ends. Here, check this out" and do whatever I felt like doing.