Juglone is most prevalent in the buds, nut hulls, and roots. To top it off, juglone isn't very soluble in water, so I think your idea of removing soil (and roots!) deep into the bed, and putting down some lining will show some improvement for a few years.

Linings will break down, or the roots will get through it. I'd suggest that building a big raised bed on top of the spot where you remove the soil, and clearing all walnut debris from the ground should help you get some results.

Or, cut the tree down and plant a fruit tree instead. :)

I agree with clasher. Cut down the tree. You can sell the wood you know. So don't just cut it down. Contact someone who will cut it properly and buy the wood. A lovely apple tree will reward you for years and years. Plus you can graft several different varieties onto it in a couple of years.

g'day,

maybe you could consider having a root barrier installed this would then protect any area on your side of the barrier, and be permanent.

len

Here is a link that might be useful: len's garden page

If it is Black Walnut the tree standing about 20 years ago was worth over $5,000 standing. But I love Black walnuts so much I would just have to find another way of getting around having it cut down.

It has to be black walnut, english walnut produces little juglone. I would dig down 3 ft, cut whatever roots are found, and install a barrier. Also, be aware that the only quick way to dissipate juglone is in the presence of abundant organic matter. So it could pay to put the barrier on the outside of the trench, and backfill with manure.
That, and some manure on top of the bed, will help break down juglone.

Long term, BW and veggies are not compatible, so i would cut the tree. Or, limit yourself to squash, beans, beets, carrots, parsnips, garlic and onion, which are tolerant.