Sit down with them as a group or individually and go over the laws against discrimination. It can even be a great idea to divide the issues into several topics and conduct a series of meetings, each devoted to a segment of discriminatory practice. People tend to have better recall when they digest information in smaller chunks.
Another way to help ensure against discrimination is to always have at least two people involved in any hiring situation. This offers a double-check system during the hiring process itself and gives you two witnesses should any legal action result.
Consider sending your managers and supervisors to workshops that deal with discrimination law and its implications. Or have them read books and articles on the subject.
And, if you have a manager who has a sizable department bereft, or relatively bereft, of females or ethnic minorities—be suspect.
Be absolutely sure your management team, including supervisors, understands the laws against discrimination and what your company policy on discrimination is, and that discrimination in your workplace won’t be tolerated.