The court heard how self-confessed sex offender Alan Green, 48, was being investigated for attacks on children when the men took the law into their own hands.
A gang of four people burst into Mr Green's home to search for a video he had filmed using his own camcorder.
The men had hoped the film - featuring an innocent schoolgirl - would be "concrete evidence" against him.
Henry Stephenson, 54, was seen swinging the hammer above his head before entering Green's flat in Splott, Cardiff.
The other gang members were Stephenson's two sons-in-law - Samuel Walker, 32, and Paul Price, 21 - along with a fourth man who is still wanted by police.
The court was told the gang targeted Mr Green after hearing he had enticed the schoolgirl into making a sex video.
Green has 30 convictions for child sex offences and was recently acquitted of charges against a three-year-old girl and boy of four.
'Severe blows'
David Essex-Williams, prosecuting, said: "The men burst in and attacked him before he could say anything.
"He was punched, kicked and struck with a metal object - the most severe blows landing on his upper body. He thought the men were going to kill him.
"The men then attacked Green's friend with the hammer."
A court heard the vigilantes later went looking for alleged paedophile Richard Holcombe.
Kicked to ground
They found Holcombe, 25, walking in a Cardiff street with his girlfriend and her five children.
Two of the men jumped from the car and kicked Holcombe to the ground in front of the children.
Henry Stephenson, 52, of Stafford Road, Grangetown, Cardiff, admitted actual bodily harm on Alan Green and Richard Holcombe and having an offensive weapon.
He was jailed for 15 months.
Samuel Walker, 34, of Westdale Road, Swansea, admitted actual bodily harm on Alan Green and having an offensive weapon.
He was jailed for 12 months.
'Ugly scene'
Paul Price, 20, of Grangetown, Cardiff, admitted actual bodily harm on Alan Green and Richard Holcombe and having an offensive weapon.
He was put on probation for two years.
Judge John Prosser said the men had been involved in an "ugly scene."
"It seems to be very fashionable that if there is a paedophile about to go and attack him.
"The sooner that everyone disabuses themselves of this idea the better it will be for society.
"They should let the police and other agencies get on with it."