I feel your pain and have this done to me quite a bit. If I see anyone with the rage in their eyes, I will come at them with my weapon to warn them not to fuck with my horse, if they hit it anyway I'll TW them teach them a lesson. Most people get the picture pretty quickly, if it doesnt belong to you, dont touch it.
Admins have not and will never be able to enforce stuff like not messing with peoples horses, so we have to defend ourselves. Just like if someone is chopping a team ladder to the flag as attackers in the final seconds of a map, I'll kill them to save my win.
I really wish warband supported in-game text macro'ing like Quake/Halflife so I could just hit a key
bind x say_team "DO NOT HIT RED HORSE, REMOUNTING MOMENTARILY"