Johnson said that cards offering "massage services" were often thinly veiled advertisements for prostitution, and linked to organised crime and violence.
"This is one of those areas where the public can have a huge influence," Johnson said. "People do not understand the deep and intimate connection between a little card in a newsagent's front window and organised crime."
The National Federation of Retail Newsagents (NFRN) said retailers should "think carefully" about the types of cards they allowed in their windows.
"For the sake of a small income that these cards provide, retailers need to consider what sort of people they want to attract to their shops," said NFRN trade relations manager David Daniel.
However, not all retailers felt that asking the public to confront store owners was the way to tackle the issue. "Telling shoppers to confront retailers could create confusion and conflict," said Mina Patel of Mace Express in Bow, London.