Blackberries are hard to get rid of. They grow on vines and even when you think you've dug them all up or cut them all down, they still grow back. They can even invade unwanted spaces. If you are done with your blackberries and wish to kill them, fortunately, you can take on this task with great success. It will take some time though and is a process that is best started in the spring when the blackberries are most active in their growth.
Cut the vine down to the ground with a pair of garden clippers. Wear gloves since blackberry vines are often prickly.
Dig up your blackberry plants and throw out the roots away from the garden area. Do this when the soil is wet to make the job much easier. This step is optional, but will help kill the blackberries more quickly.
Cut off new shoots as they grow. Keep this up and don't let your blackberries grow back. The plants will be unable to soak in the sun and convert it into usable energy. The plant will eventually die.
Apply an herbicide as an alternative to the previous steps. The herbicide should contain 2,4-Dichlorophenoxyacetic acid (2,4-D) or triclopyr and may be labeled to kill blackberry vines and bushes. Follow the application instructions on the label and repeat as necessary.