I delt with blackberry brambles in CA. My brother in law (who ran a company that sterilized rice fields) gave me a plan to rid my life forever of berry brambles.
In the fall, as the brambles are preparing to going dormant (still green but starting to turn), spray them with brush killer (mixed at regular strength). My brambles were so thick, I didn't worry about poisoning the soil. In the fall, plants are pulling all the nutrients possible down into the roots. Add a little poison and they are gone forever. It took me two years to get rid of about a 1/2 acre of brambles but they never came back.
Then hire someone to get rid of the tops - the roots are dead so won't be back. Just cutting the tops will not get rid of the problem.
