Harpoon his ass, close in and cover with the 5-inch on autoload. Cover the boarding party with 50's. That's IF anything is left.
The pirate "ships" are mainly speedboats that they use to get boarding parties onto the ships. One 250-lbs JDAM would turn one into splinters
The Predator B (the new, larger version of the old Predator) can carry 3,000 pounds of ordinance, plus 3,000 pounds of fuel, giving it a loiter time of 32 hours at 50K feet altitude. I would guess you could put a pair of 1300 lbs harpoons on one and stay within the weight limits, but I'm thinking you would need to redesign the hardpoints on the wing