It isn't training always, though sometimes that will work, but it's mostly time. They need to get past that age where, like @2ndTink said, whatever is causing their sex drive to peak. We've been there, done that, and seen cockerels bloom into wonderful roosters. We've also culled a couple along the way that were too mean to put up with as they were drawing blood off or killing another chicken. The latter actually happened, and the aggressor was culled with a 20-gauge within a minute.
How much time, though, are you willing to deal with it or try your tricks or pen him up, etc., as that can take months in many cases? We rarely hear of older roosters being too aggressive, though it happens, and those I wouldn't keep either.
