Every June a few US supreme court cases get a reputation for being blockbusters, and this year has been no different. We're still awaiting decisions on cases concerning gay marriage and the Voting Rights Act. But the blockbusters can obscure smaller cases with profound effects. On Monday, the court quietly delivered a destructive, toxic decision on workplace harassment that is as significant as anything else this year.

Vance v Ball State University, which concerned the interpretation of a section of the Civil Rights Act, shouldn't have even reached America's highest court – but it did, and the court's right wing grabbed ahold and used it to further gut workplace protections.

The petitioner was Maetta Vance, the only African-American woman working in the catering department of Ball State University in Indiana. Her supervisor, a white woman, appears to have made her work life a living hell. The supervisor assigned her to perform menial tasks, such as slicing vegetables, even though Vance had worked at the caterer for years and frequently prepared formal dinners for the university. According to Vance, she faced not only frequent racial harassment, including references to the Ku Klux Klan, but sometimes physical threats as well. On one occasion, at least, the supervisor allegedly slapped her.

Vance sued the university for permitting a hostile work environment, but there was a catch: although the harasser controlled Vance's day-to-day responsibilities at the catering department, she didn't have the power to demote or fire her.

For Sam Alito, writing for the five members of court's conservative bloc, that distinction meant that Vance had no case. Ball State can't be held liable, since the harasser wasn't really a "supervisor", only a "coworker". An employer can only be held responsible for a harasser's actions, the court ruled, if it has empowered the harasser "to take tangible employment actions against the victim" – such as demotion, a change in benefits, reassignment, or dismissal. The Equal Employment Opportunity Commission, the federal agency responsible for investigating discrimination complaints, had long espoused a more realistic understanding of how workplaces are organized, but Alito had no time anything so "nebulous".

Until Monday, the court had accepted that someone who directs an employee's daily activities is a supervisor. No more. Now, according to the court, unless your harassers have the explicit, formal power to hire and fire you, then they don't count as a supervisor – and therefore you can't bring a suit against your employer. But as Ruth Bader Ginsburg explained in an understandably exasperated dissent, joined by the other liberal justices, such an extremely narrow definition fails to account for the realities of the workplace:

"Supervisors, like the workplaces they manage, come in all shapes and sizes. Whether a pitching coach supervises his pitchers (can he demote them?), or an artistic director supervises her opera star (can she impose significantly different responsibilities?), or a law firm associate supervises the firm's paralegals (can she fire them?) are matters not susceptible to mechanical rules and on-off switches. One cannot know whether an employer has vested supervisory authority in an employee, and whether harassment is aided by that authority, without looking to the particular working relationship between the harasser and the victim."

Justice Ginsburg goes on to detail all sorts of recent harassment cases,– including one concerning a female truck driver bullied into having sex with her male colleagues, that will no longer qualify for judicial remedy under the court's new principle. The result, she concludes, is that the terrain for harassment cases "has shifted in a decidedly employer-friendly direction. This realignment will leave many harassment victims without an effective remedy and undermine Title VII's" – that is, the relevant section of the Civil Rights Act – "capacity to prevent workplace harassment".

In one way, the decision in Vance shouldn't surprise us too much. A comprehensive study published earlier this year by the Minnesota Law Review concluded that the Roberts court is the most pro-business bench since World War II. In the last three years alone the court has limited companies' exposure to class-action lawsuits, diminished corporations' responsibility for environmental damages and human rights abuses, and (most notoriously) allowed corporations to spend as much as they want in political campaigns.

Yet even by the pro-business standards of our conservative high court, Vance is a breathtakingly cruel decision – one that leaves many victims of harassment and discrimination without any legal recourse. By narrowing the definition of who counts as a supervisor, the court has let businesses and corporations off the hook for all sorts of abuses, siding once again with the powerful at the expense of the powerless.

And it means that women like Maetta Vance, who live and work in fear, will have nowhere to turn.