The info I have is that 'Black Hamberg' grapes are a spur pruning type. That means that in mid winter, you cut all of last years growth back to 2-3 buds, and maybe even thin out some of these "spurs", to keep things from getting too congested. If growth control is needed during the growing season, you can prune the long shoots back by 1/3 , as long as you leave a cluster about every 30 to 50 cm.