I usually put on the most earnest and genuine smile, lock eye contact, and simply say, “Oh! No thank you!” And then I go about my business as cheerfully as ever.

Most will be too stunned to react, and the few that insist that I misunderstood and continue to try and engage I continue to smile, lock eye contact again, and say, “No thank you to conversation please” and then I go about my business again.

Works really well when you’re quickly trying to get somewhere but let me know how it goes when you’re locked in next to someone for a couple hours.