RE: Powerful Weed Killer
2 things you can do, Keep knocking the green shoots back every time they sprout eventually without photosynthesis the roots will die (this is only if the plants roots are in your garden only) the other way is break fluid or petrol, This will kill everything but you will need to refertalize your garden with somthing like Horse Manure when you are ready to set it.
The Blackberry bushes roots are used to spread the plant, if it's an infestation from another garden you need to cut and clean out the root spurs in order to get rid of them.
Some people are born great
Others have greatness thrust upon them
, I find My greatness comes in a tub of Dianabol.