Accuse 9 Bloodsail Corsairs of mutiny, then kill them.
Before returning to Booty Bay, you should do everything you can to weaken the Bloodsail Buccaneers. Unfortunately, even as captain, you're prohibited from killing other buccaneers in cold blood. Only a Fleet Master can do that.
You can, however, put mutineers to the sword... and mutiny can be awfully hard to disprove.
Speaking of which, those Bloodsail corsairs that hang out on the lowest level of the ships have been acting awfully suspicious...