Michael Ammar had what I think is the best line to this one (similar to one previously posted): "Just be yourself, sir, and she'll disappear on her own."

Your reply should reflect what impression you would like to make with the audience. Do you want to look like a jerk who does magic? If so just use some of the above suggestions. If you want to endear yourself instantly to half the audience you might reply; I just do tricks, not miracles. Besides, she is probably the magic in your life.... Right?

Usually when I am asked this question it is after my show when people are saying what a wonderful time they had. Somebody usually jokes around and asks can you make so&so disappear?

I have that person step aside from the others and I wisper in their ear, 'Be careful what you say, I'm wearing a wire for the F.B.I.'
I then walk right away without another word, boy it mess's with their heads!

Johnny

I would but there are so few beautiful women already.

I'm sorry, but, I don't do requests.

LOL...this I LIKE.

I charge by the pound

Sorry that I can't give proper credit for this one: "Sure, this is California. For enough money I can do anything!"
Amazing Stephen

Honestly I feel the least offensinve and safest way to go is something Jeff McBride taught me a long ways back when we had hours of sessions in my living room.................as I watched his videos. At the beginning before anything he would say how he is the magician and does all kinds of things blah blah (insert your own patter) and than say I can also read your minds and no I can't give you a million dollars (look at wife) and no I can't make your wife disappear. (look at husband) This usually gets a chuckle to a roar but even if it doesn't get to much of a laugh it would be ridiculous for anyone to say that lame line for the rest of the evening.
PLus even if they do they are probably the only person not listening in the beginning and they have a good laugh at his "ignorance" than what you will say. Honestly as a young magician I CANT be rude at all when it comes to those jokes so this is my safe way out.
Hope you use it
RogerJ