Removing fig tree roots

Hi,

Over the weekend i chopped down the out of control fig tree that we inherited on moving in. From counting the annual rings it appears to be around 20 years old. It took up too much room in our smallish garden (tree was 7foot tall and about as wide), wasnt bearing fruit (Jesus cursed a fig tree to die for this reason - as google searches for advice revealed!) and just looked messy. Before i'm struck down by people saying not to cut down trees, i intend to replace it with a couple of fruit trees!

Anyways i hacked it down to the base and am now left with the roots to contend with and complicated by being a foot away from the fence, two feet from a washing line pole set in concrete and 3 feet from a wall between lawn and veggie patch. So far i've dug down to upto 18 inches all around but one of the roots passes under the fence, another under the line pole, another under the wall, and another under my newly seeded lawn (yep i know should have dug it up before the lawn but we hadnt decided to remove it then!). Typically the majority of the roots appear to go under the obstacles!

Is it ok to just cut off the roots that pass under the fence, wall and line pole? Someone suggested bashing a couple of copper nails into the cut-off roots to guarantee it won't come back!

Any advice appreciated

Andy