I'd go with Roundup.
Do you intend to grow anything there anytime soon? If so, I'd avoid salt.
Boron is supposed to kill stuff, too, in large amounts. Dissolve large amount of borax in water and pour/spray it. However, I've heard that it will prevent anything from growing there for a long time, years. Never done it myself. I think some "home remedies" can become more lethal sometimes than the product you're trying to avoid. Anything that you pour on your soil can spread out and kill unintended things, like other shrubs, flowers, or lawn. Things that you spray on foliage should have more limited effects. If you have clay soil, it can hold things for years.
Maybe your state extension office could advise you. But like I said, I'd reach for the Roundup. I always feel it works best in spring, as new tender growth emerges.
Karen
