No, then change the subject. Remember Miss Manners - you don't have to have a reason to say No, you just repeat it, nicely if you want to.

How about it?
I really can't. What did you think of the weather?
No, really, why not?
I really can't. What about the Jon Benet case?
Really, why won't you?
I really can't. Have you changed your hairstyle?

And so on.