These people can just be kicked .
That's... Not how it works
One, we'd need more admins.
Two, in case there isn't an admin on, we'd need a more responsible player base.
And three, you can just ctrl + m a player when they hit your horse to report them.
Besides, if you won't use the donkey you can just unequip it by right clicking it when choosing your equipment at the beginning of the round, and most players won't try to kill your donkey as long as you or someone else is riding it