Under employment law there is little you can do. If you are looking at harassment, it could be both a civil matter and a criminal one. The law states that a person must not pursue a course of conduct which amounts to harassment of another and which he or she knows or ought to know amounts to harassment. Although there is no definition of what specifically amounts to harassment, it would usually include alarming a person or causing them distress and must have occurred on at least two occasions.
Under criminal law, and if this is reported to the police who then take action, the punishment for harassment can be imprisonment and/or a fine. A court may also impose a restraining order for the purpose of protecting the victim.
In addition to criminal action, a civil claim can also be brought against a person who is alleged to be guilty of causing harassment. The courts would award compensation to the victim, something that is unlikely to happen if this is pursued as a criminal issue.
So in the first instance the police can be contacted and this matter reported to them as harassment. However, they will not often get involved in trivial disputes so if they believe that this is not serious enough they could refuse to help and advise you that this is a civil matter. In such circumstances, the victim can warn the harasser that their actions are being treated as harassment and that unless they refrain from such behaviour in the future they will be reported to the police and legal action under harassment legislation taken against them.
Another thing though is that as you do not have protection against unfair dismissal, you could potentially be dismissed for complaining about this or trying to take it further s be aware of this potential risk.