When I get this:
"Remember that time you...(turned the whole deck into the same card, predicted my card by turning it face-down, restored the empty can of soda, made my wife disappear, kicked me in the shins, etc.)??? Can you do that again?"
I say this:
"Yeah, but you know what... Here let's try this..."
And then I do what I want. I have found that typically their request isn't so much for that specific effect, but it is a request for you to perform for them. They may even ask you this in the middle of your routine. That's okay. You have created an impression that will always be there for them. You now get another opportunity to increase your performance value for them.
Sometimes friends/people will insist I do a certain effect. When they do I turn it around on them:
"What am I? Your personal magic prostitute?" Everyone laughs at this, including them. The next thing you know you'll have me in high heels and a wonder bra singing Karaoke to the Greatest Hits of the New Kids On the Block. Not gonna happen buddy." And then I move on, bringing attention back to whatever it was that I was going to do. It gets a laugh, eases the tension of my refusal to do the trick they requested, and allows me to perform what I had originally intended.