There is a version of Roundup that you can use on an individual leaf by leaf level. It's kind of like a large poisonous pritt-stick. It's fine for precision weeding in an otherwise sensitive bed of plants. I think you would have a hard time getting enough Bramble leaves in a hedge to have much effect, as the bramble is multi rooted, and will have multiple root sources beyond your reach. I have taken to regarding them as part and parcel of gardening, like edging the grass.