HaaRoy wrote:

I have tried cutting it hard and applying rootout crystals but it
keeps coming back.

They enjoy being hard pruned. Rootout decomposes into excellent
fertiliser too.

what will kill of this damned thing ?
sulphuric acid poured at the base ?
caustic soda ?
agent orange ?

Glyphosate is probably as good as anything, but digging the roots out
will probably work.
It is more of a vigorous shrub than a tree. I expect a combined method
to work best.
Simple acids or bases may injure you but an established tree will pretty
much laugh it off.

Regards,
Martin Brown