This is getting embarrasing ;-) Why was it banned? (besides from being neo-nazi)
The CP'86-party was the most extreme rightwing party ever to have won seats in Dutch elections, although only in municipal elections, 4 seats in 1990 and 9 in 1994. It resulted from a split in the CP (Centre Party), which won a seat in the Dutch Second Chamber in 1982. The more moderate members of the CP continued in the Centre Democrats (CD), which was quite successful in the period 1989-1996 in the elections. The extremist members of the CP united in CP'86.
Demonstrations organized by CP'86 always ended in riots. It was quite a typical neo-nazi party.
A number of prominent members of the party were prosecuted for racism and for urging on the use of violence against foreigners. In the end the leaders of the party were convicted for being "members of a criminal organisation", so the party was seen as being criminal. With this, it was easy to ban it.
However, at the time the party was banned, almost nothing was left of it. Extreme rightwing parties in the Netherlands are famous for splintering. CP'86 was no exception.
In fact, the CP'86 was the third party to have been banned in the Netherlands, in 1894 a socialist/communist party was banned and after WWII the Dutch national socialist party.
At the moment there is no strong extreme rightwing party anymore. The strongest party in that area at the moment is the NieuwRechts (NewRight) party, whose leader was kicked out of the Pim Fortuyn List, because he has ties with the Vlaams Blok. Its program may be compared with the new program of the Vlaams Blok. In the European elections of this year, they didn't score very well, only 15,000 votes (0.3%).