Street photographers demanding money for unsolicited photos at Galle Face

This is an aggressive sales tactic, but not a legally enforceable debt. If you did not ask for the photo, you are under absolutely zero obligation to pay for it. Put your hand up, say a firm "No thank you," refuse to touch the printed photo (if you touch it, they claim you bought it), and keep walking. If they harass you, walk toward the police post near the flag.