Corazon, unfortunately like the majority of people have said, there's not a lot you can do. One Question, Rusty, is he yours, what I mean by that is did you buy him or is your parents -?father - the legal owner? If your parents are the legal owners of Rusty you have a slightly bigger problem. If he is yours legally then you have every right on this earth to tell Joe to leave your horse the heck alone! If this horse is legally the property of your parents, you are going to find some eway to convince them that letting Joe or anybody not approved by yourself ride your horse is counterproductive to all the training and time you are putting into your horse. Maybe if they are paying, or helping you pay hearing that letting Joe ride rusty is detrimental to his training and thus indirectly wasting their money. If you are paying for your own lessons, talk to them like an adult, tell them please, you work hard to afford those lessons to better yourself and your horse and letting Joe ride him is undoing a lot of hard work and wasting your hard-earned money.
As for the other horses, i feel your pain, but without your parents' support there's not really anything you can do about that situation. You can try talking to your dad like an adult and tell him that not being able to feed Knight apart is wasting precious money spent on grain. Most of all, keep your cool and get your facts in a very logical order. Approach your parents as the adult you now almost are and try and find some solutions, even if you have to compromise a little, especially on the "other horses" scenario. Your parents are NOT going to listen to what you have to say if you come across even in the slightest like a spoilt brat (no offence meant, please). Lastly find a way to lock up your tack, a box, a cupboard, anything.