Hair-by-hair editing / Trees distribution / negative weights?

I’ve been using an old Particle/Hair/weight map technique to distribute tree meshes on my ‘landscape’, but occasionally a tree appears in the wrong place, eg the middle of a road.

Is there a way to single out a particular “hair” and delete it? Weight-painting is a pretty coarse control.

BTW, are there “negative weights”?

Go to Particle Edit mode and use the Cut tool to delete the particle responsible for the tree in the middle of the road.