I think some series have redefined blocking lately and that is probably what Derek mistakenly was referring to. What Hamo did was make a defensive move and traditionally, the lead driver is allowed to pick his line one time, whether that is onto a defensive line or anywhere else he chooses to drive. If, after that lead driver "decision", the following driver moves onto another line, the lead driver is not allowed to move over (off his originally selected line) to block the following car. That is called blocking and, at the very least, is not very sportsmanlike even if it is allowed by a particular series. I believe F1 allows it (probably until it's dangerous).
The obvious example that Derek was probably thinking of (and obviously agrees with), is the rule that Champ Car made this year. I think they have said that a defensive line is not allowed now and the driver's intention in this regard is then judged by race officials. If he's off the racing line, did he move there to impede a following driver or did he inadvertently get there by making a mistake? If his intention was to impede, and is a obvious attempt to dodge the intent of the rule, they will give him a penalty, a drive thru or, it that's not possible, a loss of position.
I think the Champ Car rule is a little extreme. Good fun racing has a place for sportsmanlike defensive driving (but not blocking) and is part of a good dice.