It has been said that there are at least two sides to every story. This will try to cover both sides of the sexual harassment topic. In this case there is the management side, (M), and the employee side, (E). Let us look at the factors from both sides of communication.
(M): Make clear the policy for the office regarding sexual harassment, and communicate it in a fashion that is easily understood. When in situations that might allow for a charge of harassment, be clear in what you have to say, not just in the words, but in tone, and in body language, which includes facial expression. Much can be made of the way in which something is said.
(E): If you think someone has said or done something that could be sexually harassing, tell that person straight out that you think what they have said or done is not okay with you. Let them explain. If that does no good, communicate this clearly to the next person up the chain of command from you. If the alleged perpetrator is that person, go to the next step up. Document what has happened, so the memory is fresh, and the information can be passed on without confusion.