Af first the media gave them no air time, they weren't invited to political debates, they filtered them out like they didn't exist. The political parties created a cordon sanitaire, somekind of agreement to never create a government with that party involved.
The result? The biggest win ever for a far right party. To this day that particular moment in history is called black sunday. They were the underdog, the ideal party to give a protest vote and their idea's for a lot didn't seem that ridiculous as they got no confrontations.
Later they tried to outlaw the party but it just resurfaced as a new party called the Flemish importance (Vlaams Belang). Then something changed and they got more airtime, they were invited to political debates or debates too court.
The result ? People saw how ridiculous some of their arguments were or how they lost debates when confronted with just sane arguments.... Today they are a marginally sized party at best and nobody is scared from them anymore.
The only way you can defeat those ideas is not by taking away their voice, but to let them speak.