My little roos just turned 5 weeks old today. I wonder if it's too late to start the tummy rub thing? Probably. So far, my little guys are sweet (well, most of the brabanters are kind of stand-offish, and I can't tell which of them is the roo yet) and like to use me as a roost , but who knows...
As others have said, other methods will probably work better than spanking him. Of course, seeing someone spank a rooster would be good for a lot of laughs. Also, to echo what others have also said, if the aggression is extremely bad, it may just be better to make him into tasty BBQ.