How to get rid of thorn bushes?

cut at the base and apply Roundup with a paint brush or after putting on gloves, use paper towel or scrap of cloth to apply

I do not think Ginnyd's answer will work. I just copied this from roundup web sight:

Treat the right spots When using most of our products, be sure to treat the leaves of the plants, not the soil. This lets the product work its way down to the roots of the plant and stop it at the source.

I personally have used roundup for over 35 years, and it is best put on rapidly growing LEAVES in order to translocate into the roots.

I cut them back as far down as I can, then I take a regular drill with a bit and drill one or several depending on size of stump left. Drill down as far as you can and make a hole or holes in the stump or leftover section. Then put anything you have that you wouldn't normally give plants. Keep checking and adding more. It should totally kill it, then the stump will be easier to remove. I actually put kitchen cleaner, vinegar and Coke. I was just walking by and gave it some of my coke zero. It did not harm any of the plants around it. But the cutting and drilling isn't an easy thing to do. Hope this works.

Have you used Roundup for tough vines ? Use it straight out of the bottle. Not diluted.

I am repeating, to some extent BUT Roundup MUST translocate through the leaves. If you cut them off, it can't do that. ALSO for several acres the drilling etc. would be all but impossible. THEN USE ONLY AS DIRECTED. Many hours, days, weeks of testing by the companies, determine the BEST way to use. SO USE their experience, and follow the directions. I KNOW Roundup instructions show different strenths for different weeds. I USE THEM>