While I understand the issue at hand here, I do not believe simply banning people is the solution. You have to remember people purchased this game and I can imagine it would cause many complications with Steam and cause an uproar on the community forums there. Banning should be a method which is used as a last resort and for very serious actions such as cheating or hacking. While breaking boarding by friendly players is very much a frowned upon action it is neither cheating or hacking; it's simply annoying.
I mostly believe the best solution would be to give the player initiating the battle an opportunity to lock the battle to his friends/ group. This way you avoid getting into a battle with other players who might carry out such actions as breaking boarding or stealing your boarding action on another ship.