Cut the rose bushes down to stumps. Alternatively, dig out rosebushes, or pull them with a tractor and chain.
Mow over any new shoots that grow at the site. Do this three to six times over the length of a season and continue the next season, and possibly the next, until there is no more growth. Be sure to remove all pieces of the cut bush and burn them, since roses root from cut canes.
Spray dormant weed killer over the cut and mowed bush stump.