Do you mean brambles - wild blackberries? They are very resistant things.
Cultivated ones aren't as tough and you may get rid just by serious digging. Cut them down to ground level and burn the cuttings, then dig out every little bit of root. They will resprout, you'll have to dig several times.
Or, I'd recommend a glyphosphate weedkiller, at extra strength. It's a relatively benign weedkiller, kills anything green it hits, but rapidly breaks down to become harrmless. Doesn't kill wildlife.
Give them a good dosing and wait for them to die. Remove & burn the dead wood. They will then resprout. Do it all again. Eventually, you'll get them all, but it is a drawn out affair.
Just been through all this, in a field.