Peter Cook was a passenger in a yellow Mini Cooper which was allegedly stopped by officers on the M25 yesterday for displaying the slogan.
He claims police pulled the car over for erratic driving and threatened him with five penalty points on his licence, before telling him the signage on the vehicle was an offence under a section of the Public Order Act 1986.
The Gillingham resident said he called 999 during the incident due to "concerns for his safety".
He was asked to go with the Mini driver to a Shell garage forecourt on the A12, where he asked more officers to attend to "justify the action of the policeman".
On arrival the B and O letters of the slogan were removed from the car.
While protesting the use of the word Mr Cook was reprimanded for swearing in front a mother and her child, he claims.
Speaking about the incident the Gillingham and Rainham candidate said: "The officer was very angry and refused to explain his actions nor to listen to our justifications nor to consider our legal understandings.
"I feel he played judge and jury. The event lacked proportionality given that we were not armed, carrying drugs or proved any risk to the public.
"I got no answers when I questioned his authority under the law to demand removal of the slogan, especially as the car does not belong to us.
"The signage cost £300 alone. I see the whole experience as a worrying development in our current political climate and question the police response to our freedom of speech and our democratic right to protest.”
Earlier this year Mr Cook was punched at Gillingham railway station after being taunted for wearing an anti-Brexit t-shirt.
He has also penned an album in protest against the possible split from the EU.
Essex Police have been contacted for comment.