You can't stop employees and low-level supervisors from comparing notes and speculating about's motivations; the right to complain is practically enshrined in the National Labor Relations Act.
But the good news is that you can't be held responsible for suggestions that the organization's actions are steeped in discrimination, as long as the person speculating isn't in charge of hiring, promoting or firing.
Still, it's best to remind employees with any amount of supervisory authority to avoid negative comments or speculation about the employer's actions.
Recent case: When Ruben Cardoso worked for Bosch Corp. in Brazil, he earned a substantially lower salary than Bosch employees in U.S. locations. When Cardoso relocated to the United States, he won a raise, but still earned less than his American co-workers.
When Cardoso complained about the disparity, a supervisor told Cardoso the disparity was "because you're Brazilian," even though that supervisor had no direct knowledge or control over salaries or hiring. But the comment was enough to send Cardoso to the courthouse with a national-origin lawsuit.
The court didn't bite. It dismissed the case, reasoning that the statement hadn't been made by anyone with the power to hire or fire or in a position to really know why the pay rate differed.
The statement was not direct evidence of discrimination, and Bosch countered with real reasons for the pay disparity, including education and experience levels. Idle speculation wasn't enough. (Cardoso v. Bosch, No. 04-4026, 7th Cir., 2005)