If you take over a large ship, you can't just beach it on any convenient shore and then negotiate its ransom. You have to take it into a deep water harbour and hold it there, where you can dock or anchor it, unload what it's carrying with shore cranes, and so on. To stop the piracy, therefore, you must shut down the Somalian deep water harbours and make it impossible for the pirates to use those harbours. I doubt there are very many such harbours on that long coast.