I have been trying to work out a deflector system for boids that actually works and I have achieved some success so I thought I would post the BLEND.
What I discovered is that for deflectors to work for boids already in motion towards a goal, the deflector needs to be about 10 times as strong as the attractor to have any effect. This is not mentioned in the documentation at all. To the casual users, myself included, it appears that they system does not work. However, on a whim I decided to jack the number way up and viola it actually worked.
The scene has a single boid attracted to the sphere with a field strength of 10. A cube is placed around the sphere with a field strength of -100. The max falloff is enabled for the cube, so the boid does not detect the cube until it gets very close. When it does detects the cube, the boid moves away and avoids the area until it moves out of the falloff range, then it is attracted again and repeats the process.
boid_attractor_deflector.blend (227 KB)