What is the ruling on one who unjustly and aggressively causes people's livelihoods to diminish?

Every creature has had its provision (rizq) decreed by Allah, and He alone is the Provider. No one can cut off a provision that Allah has apportioned except by His permission, in confirmation of His saying: "Whatever Allah grants to people of mercy, none can withhold it; and whatever He withholds, none can release it thereafter."

As for intervention to reduce a salary: if it is done to cause hardship to the employee out of envy, or if the work truly merits the salary, then it is an injustice. However, if the salary is genuinely more than what the employee deserves, and someone intervenes to reduce it as advice to the employer, then there is no injustice in this.