I'd avoid eye contact first off. Then they inevitably ask, I make eye contact with no smile or facial expression, say "surgery" real dead-pan, and then break eye contact. I'd do this while moving. And if I was stopped and someone asked, I'd do exactly the same thing but start moving away from them.

When I'm in the "dont bother me" mode, I find that any explanation I give people just invites them to ask more questions. The less words I use, the less they have to jump on me and bother me with.

oh, and wearing sunglasses or a set of headphones works wonders. even if I'm not listening to anything. I call these "avoidance shields" LOL!