Kalista's Oath/Ult cannot be disabled

There needs to be a way for the recipients of an unwanted "Oath" to disable it, or to force Kalista to pick a new target. I was just in an ARAM game with a Kalista, and the Kalista player selected me as "Oathsworn" without asking first. I asked repeatedly and politely, and the Kalista player kept using the ult on me.

Having your character vanish off the field of battle in the middle of combat, your UI changed to a weird overlay color, and being forced to fling your well-positioned champ someplace else, is not a pleasant experience.

The Kalista player claimed he/she could not change who Kalista was bound to, but still refused to stop using the very troll ult.

I reported the player, but this needs to be fixed. It was an unpleasant game experience, and if it happens again I'll probably just take the hit to my score and AFK instead of having 20 minutes wasted by a troll.