Walk the Plank!?!

[CaptainGreenGene]

First, unless you accept their friend requests they can't get onto your ship. If you added them to your friends list that amounts to "opening the door" for them to board your ship. Port your ship and delete them from your friends list. In the future don't accept friend requests from strangers..

There is another way players can board your ship. There is a glitch that allows players to walk off their ship and over to yours. They can't use the wheel or the cannons but they are clearly visible on your ship. You can click on them and use the report button. No further action is needed. When you sail away they fall off your ship so if this happens just sail away from their ship and they plunge to the sea.