Chris, the secret service outfit sounds good. I would add
a camera or two, so as to take every brown-shirts picture.

There is probably a local nazis cell and some of the marchers
are local. They need to be brought out in the light. Maybe the
people in your community will then help break up the cell.

Another more passive demonstration would be 'pity' signs
during the march. There is so much hate in their hearts
that there is no room for Love.

Like....

"Sorry you have no love in your heart" or

"Life is the pits without love"

Dennis