Essentially, I'm in agreement with Ziggy and Skander here.
Sometimes, a player simply CANNOT make a play on the ball. Those are the times that, as a sportsman, you have to either concede the lay-up, or commit a foul in a way that you won't hurt the player but will stop the play.
Ariza did neither. He chose, as is so often the case with flagrant fouls, to stop the play NO MATTER WHAT. In what was essentially a blow-out game, THAT SHOULD NEVER HAPPEN. It's called SITUATIONAL AWARENESS.
Sure, like any basketball fan, I want to see players that play hard, or that get drummed out of the league for not playing hard (which should have been the result for Stephon Marbury). And sure, I don't want to see players quit on plays. But at the same time, if that's Dwight Howard going up for a dunk with two hands, and Ariza is coming TOWARD him instead of blindly behind, Ariza ain't going up for a block. I expect the same kind of awareness to consequences when your play means that you'll be hurting a player as you'd have when it's your own body you're protecting.
I've yet to see ANYONE make a block like the one Ariza was trying to make. Prince got a clean angle. Ditto with a couple of Bobby Jones plays. Or Olajuwon vs. Strickland. NONE of those involved a player flailing wildly at an opponent - especially in the vicinity of a player's HEAD. On the other hand, pretty much every wild flail that connected with anything resulted in a flagrant foul.