A scheme between the council's trading standards department and banks in Croydon has prevented a vulnerable pensioner being conned out of more than £50,000.
The 89-year-old man had already lost £100,000 after falling victim to two earlier scams.
Rogue traders visited the man's home in Shirley and panicked him into agreeing to allow them to carry out work to his chimneys, assuring him they were in imminent danger of collapse.
No quote was offered for the work, but after reducing the height of the four stacks, they demanded a bank transfer of £52,500.
When a British Gas technician visited the house days later to carry out maintenance work, he discovered the cowboy builders had dismantled an important flue, rending use of the resident's gas appliances unsafe.
The gas supply was subsequently disconnected before any harm as done.
Croydon's trading standards team were aware of the victim because of two incidents in 2004 and 2010, in which rogue traders had conned him out of £100,000.
As a result they had liaised with his bank, asking staff to raise the alarm if large sums of money were withdrawn from his account.
The latest request was noted and bank staff alerted the team after delaying the cash transfer.
The team who are now investigating, spoke to the man to make him aware he was being defrauded and to ensuring he paid no money to the criminals.
Councillor Simon Hoar, cabinet member for community safety, said: "This is a case in which, once again, heartless, cowardly criminals have identified a likely victim and moved with ruthless intent.
"However, thanks to the work our trading standards unit has done with local banks, the scam failed.
"The fact that this is the third time he has been targeted makes one suspect that these criminals – because that’s what they are – are sharing his details with other villains and he is being repeatedly targeted.
"We must all be vigilant and remember to never entertain traders who turn up uninvited on the doorstep, offering to do work or provide a service. Politely tell them ‘no, thank-you’, close the door and immediately report the incident."
Suspicious doorstep callers should be reported to Citizens Advice Consumer Service (formally Consumer Direct) on 08454 040506.
Residents who feel threatened by the attitude or actions of doorstep callers should call the police.