I've never had success nipping bad behavior in the bud once it's begun. No methods worked in any of my cases....if anything, this exacerbated the issue. In fact, as explained below, my technique in raising them was likely the culprit for repeated belligerent displays.
Logically speaking, it makes sense to me that a rooster shouldn't "fear" their handlers, as is common practice, lest they feel obligated to react out of anxiety. Looking back, my instructed terrorization of young cockerels (starting before they reached maturity) may be to blame for rock bottom success rate.

Only 1 out of 30/40-some retained unaggressive qualities....terrible odds. That's a mistake that will never happen again. Deliberate invasion of his space, pinning him to the ground for instinctively crowing in my presence, shoving him roughly (baseless, too) out of the way, stomping loudly towards him ect., seem to teach fright, rather than respect. These also did not stop a cockerel from flogging, instead egged him on.
I quit following the pack, so to speak, and started doing things
my way. Last group of 14 cockerels and their female hatchmates received equal treatment, with wonderful results. Hand feeding and affectionate socialization worked wonders. Today, 6 are model young men. (before creating confusion, only 3 of 14 attacked us....the rest were eliminated not for hostility, but rough tendencies towards the hens)
Lesson learned. Never again will I scare a cockerel.
~Alex