Adverts tied to lamp-posts resulted in the prosecution of an agency and its director.
The adverts were for a lettings clinic being held by Simpson & Partners.
The firm was found guilty of illegal advertising at Northampton Magistrates Court, where director Andrew Simpson pleaded guilty to fly-posting.
The court ordered fines of £1,500, plus costs of £1,080 and a victim surcharge of £120.
Paul Bland, head of planning services, said: “An investigation, following a complaint from a member of the public, confirmed that this company had broken the law by illegally advertising its business.
“Enforcement officers had spoken with representatives of the business on a previous occasion to explain the laws and consequences of fly-posting but these warnings had been ignored and so we pursued a prosecution.”