This is called, in the US anyway, a Strategic Lawsuit against Public Participation. I call it ‘legal terrorism’ because the whole point of it (and the company does not necessarily have to have any chance of winning its case) is to deter people from speaking out through the threat of legal costs and the general disruption and anxiety of being involved in a lawsuit. This is especially significant in England because the cost of legal representation is so high, and the laws are so much on the side of the plaintiffs, that even the mainstream media give into lawsuits that have no merit, particularly libel cases. Some US states have provided legal channels to strike down SLAPPs as SLAPPs deter people from exercising their rights of free speech.
George Monbiot has more on this here, and also points out that the public can inflict unintended consequences on companies that do this.