The person harassing may not be aware that they are behaving in a way that you find offensive. If you do or say nothing, the person may assume that you do not object and may continue.
Make your feelings known clearly as soon as the problem starts. Be assertive, point out their behaviour, describe your feelings and ask them to stop. Make sure your body language confirms what you're saying, Never feel intimidated, pressured or threatened into doing anything against your will.
Learn to assess people mostly by their behaviour and mannerisms. Observing a person's body language is the most reliable way of detecting someone's real intentions.
If you feel you maybe or have been verbally, physically or sexually harassed then it is important you use your safety network of people for support and talk to someone you can trust.