Barring bullshit (sliding into home plate, teleporting, ground collision), which does happen, if you can not kill a downed rider then maybe you should just run. You should certainly stop posting, because any cavalry that gets close enough to hit someone is usually in range to have their head taken off.
If you really hate cav that much, you are probably one of those stupid assholes who think horses dissappear after the first round, call the medic and get a derectocraniotomy.