Ground Clear will kill it out for 12 months, Commercial RoundUp, not the watered down crap from the big box stores, will kill it out too. You can get it at some farm stores or through some landscapers that sell retail sod and such.

Mix it with diesel fuel instead of water if you want it really dead. These are petrol chemicals anyway.

When I want all vegetation removed for a long time I go for the Diesel fuel.

Some of the off-brand glyphsate jugs are better than others and it's hit or miss what you get. I've bought some that wouldn't kill at 2x rate and some that scorched the earth.
In general, I won't buy Round-up (brand) at less than a gallon @41%, you're getting a huge retail markup for less or a less concentrated product. Round-up Pro in 2.5 gal was worth the up-charge when it was on sale for ~$80 vs generics. I suspect the Lesco product on the previous page is the same formula (Glyphsate + surfactant).
If you have a forklift, the best is a bulk caddy.