The leader of the far-right group, Britain First, has been found guilty of harassment.
Paul Golding was arrested after trying to approach Sajeel Shahid, who was alleged to have been involved in the training of the London 7/7 bombers.
Instead he got the address wrong - and harassed Shahid's sister-in-law instead.
Paul Golding was found guilty of harassing the sister of a man linked to the 7/7 bombings
Golding was also found guilty of wearing a political uniform, which is an offence under the Public Order Act 1936, The Essex Chronicle reported.
At Chelmsford Magistrates' Court, District Judge David Woollard said: "It was a political stunt which was designed to further the cause of the party and to generate the kind of material which is later placed on the Britain First website."
Golding was fined £425 in total and ordered to pay £532 costs.
Afterwards, Golding called the trial a "foregone conclusion". Standing in front of around 12 Britain First members, he thanked "all of them" some of whom had travelled from "as far north as Yorkshire".
He added: "We live now in a country where Anjem Choudry can stand up in court and win the right to preach in public despite terrorism convictions.
"I knocked on a door and spoke to someone and I get and yet I'm convicted of harassment."
Yes Paul, that is the difference between free speech and harassment.
He then went on a bit of a weird rant about women answering doors, while tacitly acknowledging that he and his group Britain First appear thuggish.
He said: "For some reason, every time we knock on [supposed Islamists'] doors, their wives are sent to their front doors. It is ridiculous in the extreme.
"If there was a group of people outside my front door I would go and answer it like a real man should.
"These people are despicable cowards."
Sure Paul, they are cowards - just like the groups of men aggressively knocking on the doors of people's homes, where children are also likely to live.
Golding ends by reiterating Britain First's goals of "preventing Islamic extremism and Islamic terrorism."
Just like MI5 already do. Only with more people. And more funding. And way more intelligence.