Bullying is a serious threat to an organisation's ability to innovate, attract, leverage and retain talent. It is a leadership, HR and social responsibility challenge. This brilliant video is a great introduction to this insidious problem.
It is important to recognise that bullying is not a matter of disagreement or personality conduct, but rather a matter of abuse where the abuser (bully) perpetrates a set of actions against their target.
Mediation does not work! Bullying must be treated as a misdemeanour or misconduct; and as such requires factual investigation and documentation.