Let 'em talk. The best defense are the facts.

Here in the States, at a KKK rally recently, there were counter demonstrators dressed in clown suits. Whenever the cry of "white power" came up, the demonstrators shouted "white flour" and threw it into the air. One KKK'er got mad and went after a demonstrator. A cop nailed the KKK guy and arrested him.

Humor is a great weapon. BNP can stand for "bollocks new position" or something like that.
Write some songs about the BNP and their tin pot soldiering.

The BNP and the BJP have roots in common.