So, playing a game, tense moment with evenly matched armies shifting around d each other, spells queued up to shift the balance, in comes my vassal, with a single hero and some crap unit, a 2 stack, eagerly committing suicide and DRAGGING TWO OF MY STACKS WITH IT into an unwinnable combat. If we must have free cities suiciding into the enemy, at least don't make me have to save scum to avoid committing suicide with them. A simple "deny assist" button would be so, SO easy.
Also they are my vassal why can't I tell them not to attack. War is my job, paying tribute is theirs