if the game can know that "a summoner has disconnected" then it should know that a kalista player is without an ult, in this case the black spear active portion should become available again so u can use on another person. And if the original Soulbound reconnects, then the second Soulbound is automatically not a soulbund any more.
I think this is the only case that you should ever get another black spear, or maybe that person knows that they have to go (shame on that person for not managing their time correctly). In this case they can notify Kalista , if they have to suddenly go. There can be a mechanic, where Kalista initiates the withdrawal of the black spear, and the Soulbound clicks accept, this works if you Black spear stab b the wrong person too i guess.