Why Can Kalista Rebind?

When Kalista is bound to someone, they stay bound with her. It's how her game mechanic is supposed to work. If twenty minutes pass and the person Kalista is bound to AFKs/Quits/Disconnects, you cannot rebind, even if you have a new Black Spear.

Can you cancel Kalista bind?

Kalista starts a game with an unique item, called "Black spear", which enables her to bind a single ally champion for the rest of the game. ... Kalista cannot cancel her autoattacks.

How does Kalista Black Spear work?

Passive: When Kalista is near her Oathsworn, she gains +[0]% Attack Speed. If they both basic attack the same target, she deals 5/7.5/10/12.5/15% of their max health as magic damage. 10 second cooldown per target. It allows Kalista take out her frustration by physically killing the support.