Well, you can adjudicate physical blocks as a grapple. But a chain of spirit could well be a mental block, in which case the troll's strength would be useless against it. And the troll being non-human, the Laws of Magic do not apply.
The biggest advantage of wizards is that they can adapt to their targets. Enemy is big and tough? Attack or block mentally vs their Discipline. Enemy is a speedster? Crush them with main force vs Might or Endurance. Enemy is smart and wily? Blast to bits with bolts of energy vs Athletics.
This is super important to remember both as a GM and as a player. If your monster has a weakness, it can be exploited. If your character has a weakness, it can be exploited too.
The Laws protect against a lot (mentally attacking humans, etc) but they often don't apply to non-humans (what is considered non-human for this is up to debate both in and out of the game, especially in gray areas like WCVs).
In a mano e mano fight, a wizard will run out of juice before the troll if he's constantly blocking actions like that (although prolonging the spell can make this a bit moot). In a group vs single monster fight, this is bad for the monster. In a group vs group fight, this can be bad for the wizard as it eats up his actions.
Knowing the weaknesses and how to exploit them is important. Have your clever villains do this to the players if the players do this to monsters often. It's okay. It's good tactics.
On an aside, when I've GM'd games, I try to match my players tactical level (only occasionally upping or lowering them for flavor and variety). When I play, I play at a high level (I don't want my character to die), but expect the same from my GM (who wants my character to be challenged).
For instance, if the Wizard is reasonably well built for combat, he's probably good mentally (spellcasting skills do this) and in a physical fight (evocation allows you to essentially use mental skills in physical combat). How is he socially (although depending on his creativity and character, he may find a way to use spells to boost this)? How is his endurance (evocation is taxing)? If you push these buttons, things go in a different direction.
Now, on the original topic: The block can indeed block all offensive actions on the part of the troll. The troll can use Might to beat it completely (which if the spell is prolonged is the correct response), but if a new spell is cast every turn then the troll has to roll higher than the block with its attack in order to get through it. However, the wizard is also tied up the whole time (and thus can't do anything else).
With the broken leg aspect, remember that your group only gets one free tag. After that they are paying for it with FP. If it is compelled for effect, this means the troll gets a FP (that it can use to invoke aspects to help it break the block).
If the character it trying to freeze the troll's leg as an attack, the consequences taken should be in line with the attack (but the troll/GM gets to choose when to take the consequence and what consequence is taken). If the character wants to lay an aspect on the troll, that's fine. However, this must be done as a maneuver (and thus doesn't deal stress). Of course, it could also be a block (against movement so vs. Athletics).
