In what situations do people you don't know ask you what you do where they actually care what you respond? And why not just say that you don't want to say? Among the online friends I have anonymity is the norm.
On the other hand, I used to be in the habit of responding with some kind of bullshit, ideally at least a little suspicious but just barely plausible enough to prevent the other party from calling you on it right away. This always resulted in much more fun conversations than talking about my actual work with strangers.
Simply refusing to answer is just not socially acceptable outside of venues where anonymity is already the norm, e.g. online and IRL hacker cons.