You could either wax your back every month or you could go and see a beauty consultant and get the hair permanently removed with laser treatments. It could take a few visits and cost a lot of money, but it will work.
I use a spray on nair. It comes as a foam spray. It cost's somewhere between 5 and 10 dollars at drugstores. Simply spray it on, wait 5 minutes, then shower to rinse it off. You will have to repeat this process once or twice a month for best results.