How to kill roots of pomegranate trees

are there any other plants involved?

I get all vegetation killer and cut the branch coming up at a angle as much as possible and spray the killer on the exposed part where you cut so it can suck the poison down into the other roots. Hopefully that will solve your problem

Drill holes in them and drown repeatedly in vinegar. Will take a while, but it will eventually rot out.

Rock salt on the roots.

I wouldn't use salt on the roots. You won't be able to grow anything else there then, maybe not ever.

Use a Roundup type product, or just keep hacking away until no new tree growth survives.

Yes and then put more salt down after watering. It will take a few times but it works.

http://homeguides.sfgate.com/natural-ways-rid-tree-roots-43781.html