We have a similiar problem on my 'drummers' forum, best policy is to ignore them, they soon go away and you can normally spot the same person when they use many logins under different names. The are known as 'Internet trolls' and very common on internet forums, especially :-
From Wikipedia, the free encyclopedia.
In the context of the Internet, a troll is a message that is inflammatory or hostile, which by effect or design causes a disruptions in discourse, or a person posting such messages. Trolling can be described as a breaching experiment, which, because of the use of an alternate persona, allows for normal social boundaries and rules of etiquette to be tested or otherwise broken, without serious consequences.
Self-proclaimed "trolls" may style themselves as devil's advocates, gadflies or "culture jammers," challenging the dominant discourse and assumptions of forum discussions in an attempt to break the status quo of groupthink. Critics have claimed that genuine "devil's advocates" generally identify themselves as such out of respect for etiquette and courtesy, while trolls may dismiss etiquette and courtesy altogether.
The contemporary use of the term first appeared on Usenet groups in the early 1990s. It is widely thought to be a diminutive of the phrase "trolling for suckers," itself derived from the sports-fishing technique of trolling.
The word likely gained currency because of its conveniently apt second meaning, drawn from the "trolls" portrayed in Scandinavian folklore and children's tales, which are often ugly, obnoxious creatures bent on wickedness and mischief.
As a pejorative, the term "troll" may also be a slander of opponents in heated debates, a tactic often used by trolls and non-trolls. Many times a person will post a sincere message that they are emotionally sensitive about and trolls know that the easiest way to upset them is to falsely claim that they are a troll. On other occasions a person may not instantly understand or fit into the social norms of a forum where most people are the same - and so acting just slightly out of social norms, often unintentionally, for legitimate reasons gets the poster falsely called a troll. Sometimes persons who merely want to be funny are accused of trolling, when that is not their intent. Many trolls now find that the traditional trolling tactics are so overused and commonplace that they have to disguise their trolling to make it effective - although, quite often, the disguising merely involves falsely accusing others of being trolls themselves.