Based on the book text you pasted, I think they mean you need more power and corresponding control. After all, the Power of a spell is equivalent to the "roll" from another action, with the exception of Attack spells. So other than with Attacks, trying to get a Maneuver or Block off when your spellcasting is Blocked means you've got to set the Power to meet or beat the Block on you. So how do we deal with Attacks?
At the end of the day...a Block prevents action from occurring. Sometimes that means "you spit bullets and they bounce off my shield," and sometimes that means "I spit bullets at the doorway, and no one is going to go near that doorway now to escape." In the latter case, if you're trying to Sprint through that doorway and you don't beat the Block, it isn't like you run into the doorway, dance around lead in the air, and come back--you just don't go for it. (Or, you know, a million other options, but stay with me: )
So. Blocking the use of magic. I think this makes perfect sense. It isn't infrequent for Harry to wind up too up-close to a Bad Guy to get a spell off...that screams a Fists or Weapons Block. Sure, some of that is compelled, but not all of it has to be. Or, for example, say that someone with some high Athletics and Speed powers is dueling a wizard in an all-out brawl. There are bystanders on the left and right side of their "arena." So the speedster runs around to place himself between the wizard and bystanders and keeps bouncing around--if the wizard misses, he's got a really good chance at hitting an onlooker, and we can't have that; ~Athletics Block against ranged attacks including evocation. Seems reasonable to me. He could compel this as well, sure, but...the Block should work too if that's what he wants. We can argue over whether that's a winning strategy for the speedster, but it seems like a valid strategy, at least.
So...how to handle this. The idea is that generally these Blocks makes it disadvantageous for a wizard to make the attempt moreso than make it impossible to get the spell itself off (I'll get to that). So in these instances...you don't know if you'll beat the block until you roll. Meaning you've got to call up power first, and then we get into the questions we have here. So you call up power, deciding on an evocation element and some kind of attack spell...you roll to control/aim it at the Speedster, and the roll is less than the Block value...what happens? Well, I recall when discussing adjudicating Athletics rolls that if your PC makes an Athletics roll to jump across a chasm, for instance, and fails the roll...they don't have to actually jump and fall; they realize that they won't make the jump and need to find another way across. In that vein, I think what I would say is that the power you call up doesn't just disappear--you took stress and have X shifts of power called up to work with still...but that you have to decide on an alternative thing to do with the power you called up, with what is allowed being what fits in the narrative. In this case, maybe you hit the ground beneath the Speedster or the roof above him, or Block him, or whatever...you just can't throw that spell at him in a straight line from A to B, because you're not confident enough on your aim.
Or, you can make the control roll and see if it fails...and if it does, just don't take any stress and say it's because you never called any up, because you weren't confident enough that you could hit the guy without hitting a bystander. I think I'd allow the player to decide--forget the stress hit from Power and accomplish nothing, or take it but redirect the spell in another way. A third option, if you're roll-happy, is to forget the stress hit from Power but then allow them to make a completely different mundane action after they failed the Control check, but that feels like double-dipping in a way other PCs can't do, so I'm not a fan of that.
As far as more...personal blocks, like grappling....I think I'd say the same thing. Figure out the spell but hold off on marking off Power stress. Make the Control roll. Will you beat the Block? If yes, then mark off the Power stress and adjudicate the spell. If no, then no Power is called up, no stress hit, but you lose your attempt at another kind of action.
^ The above ideas are all just for Attack spells because of the different Power/Control relationship they have. For Blocks or Maneuvers, you need to just worry about Power meeting/exceeding the Block, and the Control roll is a bookkeeping roll for yourself. If you fail Control and allow enough Backlash that the Power isn't enough to beat the Block anymore, that's your issue, and the Block holds.