Amy Dalla Mura was banned from entering the Broxtowe constituency at Westminster Magistrates' Court on Wednesday.
The 56-year-old, who goes by the name Based Amy online, has joined numerous protests with the pro-Brexit “yellow vests” and far-right groups.
But the court heard she specifically started targeting Ms Soubry this year, as the former Conservative MP vocally opposed Brexit and joined Change UK.
On 23 January, Dalla Mura disrupted an event where Ms Soubry was speaking at a hotel in London, shouting over her while broadcasting the incident on Facebook.
Dalla Mura had to be escorted from the premises by security before the event could continue.
Then on 14 March, Dalla Mura approached Ms Soubry while she was giving a television interview in parliament’s central lobby, calling her a “traitor”.
BBC's Newsnight presenter Nicholas Watt, who was interviewing Ms Soubry, said Dalla Mura “looked troubled, very anxious and angry”.
"Ms Soubry looked very alarmed by this very hostile presence,” he added.
Police said Dalla Mura refused to stop filming and officers were called after she returned a second time.
The following week, the defendant attempted to intercept Ms Soubry outside the Cabinet Office and said online that she was going to “have a word” with the MP. But she had already left.
Detectives from Scotland Yard’s parliamentary liaison unit, which was established after the murder of Jo Cox, said Dalla Mura refused several voluntary interviews but handed herself into a London police station in September after an arrest warrant was issued.
Dalla Mura, of Eton Villas in Hove, pleaded not guilty to three counts of harassment but was found guilty on Wednesday.
Passing verdict, chief magistrate Emma Arbuthnot said she “knew that she had done something wrong and thought that she had got away with it”.
Describing Dalla Mura's behaviour as “oppressive and unacceptable”, the magistrate said her conduct was “driven by anger at Ms Soubry's political views on Brexit” and that she had “caused harassment in the sense of alarm and distress”.
Ms Arbuthnot released her on bail ahead of a sentencing hearing on 16 December – four days after the general election.
She ordered Dalla Mura to stay away from the Broxtowe constituency, in Nottinghamshire, as a condition of her bail, and said she could criticise party policies but not mention Ms Soubry by name.
The magistrate asked for a psychiatric report on Dalla Mura, who was handed an antisocial behaviour order in 2007 for repeatedly throwing herself into the sea.
Electoral Commission records show she has formally registered to stand for the nationalist English Democrats in Broxtowe.
The party, which vows to “end multiculturalism and political correctness”, called Dalla Mura a “persecuted patriot”.
She frequently broadcasts her protest activities online, operating several social media accounts including a Telegram channel that features doctored photographs of Ms Soubry and voices support for Tommy Robinson.
In an unrelated incident in June, Dalla Mura was arrested on suspicion of criminal damage after deflating one of the “Trump baby” blimps flown by people protesting the US president’s visit.
She is not the first person to appear in court for harassing Ms Soubry.
Last week, a man who sent her a “sickening” note suggesting she would be murdered was jailed for 12 months.
Alden Bryce Barlow, 55, from Doncaster, sent the post, which read “Cox was first you are next” to the MP last month.
In July, prominent “yellow vest” James Goddard pleaded guilty to harassing Ms Soubry in Westminster.
He was given a suspended prison sentence following chaotic court hearings where his supporters disrupted proceedings.
Last week, Ms Soubry told The Independent the increase in abuse against MPs was “hugely concerning”.
“What’s happened is that Brexit lifted a stone and a lot of very unpleasant things have crawled out,” she added.
“They were always there, but my real concern is the level of debate and language used has dropped to a level we’ve never seen before.”
The latest case came after police released new safety advice for election candidates as part of increased security measures.
The guidance calls for candidates not to campaign alone for their own safety and to take “active steps” to prevent themselves, as police forces across the country hold briefings.
Dame Cressida Dick, the Metropolitan Police commissioner, has described the scale and ferocity of the attacks on MPs as “extraordinary”, saying “polarised opinion” on political and social issues was having an impact.
Commander Adrian Usher, who is responsible for the Parliamentary Liaison and Investigation Team, said they would “deal robustly” with harassment and abuse.
“Abuse or intimidation of MPs, or candidates during elections has serious implications for individuals and for our democracy,” he added.
“We encourage candidates to immediately report any security concerns to their local police force in order to keep themselves, their staff, families, and members of the public attending surgeries safe.
“Strong political opinions are no excuse for abuse, harassment or intimidation of members of the public or parliamentarians. Police will treat such allegations seriously and seek to bring offenders to justice – as today’s conviction demonstrates.”