Frustrate their knavish tricks, indeed, although knavish is not the first word I would use. I wonder what happens to terrorists who are taken into custody by the Ugandan government. Do they get comfy cells, all the Korans they want, a Halal diet and their own chaplains? Or are they perhaps made a little uncomfortable.
Knavish had a much stronger meaning in the days when that song was written. It was a very offensive term.