What I do as soon as I discover I have a cockerel (a rooster under 1 year of age) is stop handling it. When it's old enough to be out of the brooder and in the coop, run, and free ranging is to make sure it knows I am the boss. I do not act aggressively toward them. I don't kick at them, dangle them upside down or any other such thing. What I do is, walk through them if I want to go somewhere and they are in my way. I keep walking and make sure they move. If one thinks it wants to challenge me, I go on the offensive. I walk or run toward it, chasing it around a bit. I move them away from the food if they're eating, just because I can. I move them out of the coop just because. If one gives me the stink eye, I stare right back and move toward him. I haven't had an aggressive rooster since I have started raising them this way. It's easier if you can start when they're young instead of waiting until they start acting up.