Yes, Roundup is good, as is any weedkiller with glyphosate in it. Wait until the brambles are growing and spray as many leaves as possible. Avoid getting the chemical on other plants as it doesn't know what is a weed and what isn't! You may have to give it 2 or 3 go's over the summer. Then when the stems are dead cut them off as low down as you can. Watch out for the prickles!