It is possible for the cable to be very stuck - notice how flattened it is where it's clamped, rather than round, which is the shape of the hole it is going through. Make sure you have backed off the nut quite a ways and then use a screwdriver to push the other end away from the brake arm. You also need to look at the mechanics of what you are doing. If you just pull on the cable it's not going to move in relation to the clamp/arm - you have to also push up on the arm as well or squeeze the brake together by hand.