Police appear to be powerless to bring charges against people who distribute flyers warning potential informants against helping detectives.
Scotland Yard is trying to find those behind "Stop Snitching" leaflets posted on the Pelican Estate in Peckham, south-east London, where 17-year-old Sylvester Akapalara was killed last month.
But the Metropolitan Police's Acting Commissioner Tim Godwin said: "I think it would be quite difficult to identify criminal offences within it.
"There may be stuff we can do in civil law, but I think it's more about having a chat with them."
Three teenagers have been charged with Sylvester's murder.