I would, using the ripper tool, cut every 5th or so stitch on the back side. Continue on for 12 to 18". Then pull the other side of the thread and it should pull out. You could start and stop at any point. I do this if it is a lot. That way you are not picking out each stitch. If you still have the quilt on the frame, then start with the top portion. If you could pull up the back thread when you start, then you can pull it after you have cut the thread on the top. I know it is not what we care to do but sometimes it is necessary. Be patient with it.