Ok, well we need to change our tactics with these pirates.
Every boat taken over by pirates needs to be stormed by force and the pirates killed. Do not give them money, do not concede in any way. They will soon learn that taking over a boat and then being killed isn't a good way to live, and hopefully they'll stop.
There can be NOTHING positive for the pirates. It must result in death for every pirate. If the owners of the boats are killed, oh well. It sucks, but giving them money ensures the continuation of the pirates behavior. Piracy must result in death, period.
That will make them think twice.