Mix paprika, sugar, cayenne, 1 tablespoon salt and 1 tablespoon pepper together, then rub mixture evenly over ribs. Arrange ribs upright in slow cooker, with meaty sides facing outward. Pour barbecue sauce over ribs, cover, and cook until ribs are tender, 6 to 8 hours on low.
Position over rack 10 inches from broiler element and heat broiler. Place wire rack in aluminum foil-lined rimmed baking sheet and coat with vegetable oil spray. Carefully transfer ribs, meaty side down, to prepared baking sheet and tent with foil. Let braising liquid settle for 5 minutes, then remove fat from surface using large spoon.
Strain braising liquid into medium saucepan and simmer until thickened and measures 2 cups, 15 to 20 minutes. Season with salt and pepper to taste.
Brush ribs with some sauce and broil until ribs start to brown, 2 to 4 minutes. Flip ribs over, brush with more sauce, and continue to broil until ribs are well browned and sticky, 9 to 12 minutes longer, brushing with additional sauce every few minutes.
Transfer ribs to cutting board with foil, and let rest for 10 minutes. Serve with remaining sauce.