The vandal attempted to pull down a plastic screen fitted to protect the graffiti, named Season’s Greetings.
But a security guard posted to watch over the work chased the culprit away.
“Some drunk halfwit has tried to pull the fencing down and the protection glazing at the Banksy artwork,” Gary Owen posted on the local Facebook page he runs. “This art is for Port Talbot, Neath and surrounding areas. We do not want it wrecked.”
The mural – painted on a garage in a nondescript residential street – was not damaged in the incident, which occurred at 10.30pm on Saturday night. Police were called but did not attend.
“It’s amazing and such an honour that Banksy chose to come and paint his latest piece in Port Talbot.” added Mr Owen. “We should be treasuring this privilege and it’s very sad that some people want to spoil it for everyone and give Port Talbot a bad name.
“I do fear it’ll become a target for some idiot who wants to make a name for themselves – and that’s sad.”
The attack came shortly after it was revealed the Hollywood actor Michael Sheen had contributed towards the cost of protecting the mural – which depicts a girl playing in falling fire ash.
A spokeswoman for The Damned United star, who grew up in Baglan, the next village along, confirmed his contribution, which included a donation to the plastic screen covering the artwork.
The screen was fitted by a local businessman.
Ian Lewis, a steelworker at Port Talbot’s steel plant who owns the garage where the work appeared on Tuesday, said earlier this week he has not slept properly since the piece appeared.
He said he was paying out of his own pocket for the security team to guard the work.
The 55-year-old said on Thursday: “It started off as fun, but now it’s gone to stress. It’s down to not sleeping, but now I’ve got the security there it’s taken that weight off me. They will stay there indefinitely now I think.”
Volunteers monitoring the mural – which shows a girl standing in falling ash from a fire – say more than 2,000 people have visited it since it appeared.
On Friday, Neath Port Talbot Council announced it was going to place two members of staff at the site to help manage the large number of vehicles which are parking and filling the side street.
A spokesman said: “This will be a temporary measure over the Christmas and New Year holiday period because we believe, due to worldwide publicity, this area of Port Talbot is going to attract a large number of visitors.
“We understand the excitement but we want to remind visitors this is a residential area and would ask that people coming to photograph or view the Banksy to do their best not disturb those living nearby.”
Banksy confirmed the image was his when he posted a video on his Instagram account on Wednesday.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies