Okay, I see where the confusion came in. A Block prevents an action from succeeding, by the rules, which is where you're coming from. Some of us, or at least me, internally processed that as preventing it from happening. And while the latter interpretation is one way to look at the application of a Block, particularly Grapples, it is not the only way. My bad.
That being said, in my opinion, all that does is shift it from a 4th law outright violation to a 3rd law grey area. Truth is a complicated issue, often tied up in the beliefs, biases and limited knowledge of the teller. For you to affect lies, and only lies, through your directed will, you will have to already know that the statement is a lie. It's really a chicken and egg question.
Now, if you believe there is a big shiny unadulterated Truth with a capital T somewhere, then it's likely the above does not apply. But Harry has stated that magic needs to be directed by will and focus, and that says to me it needs some kind of "target." Without a big Beacon o' Truth, the target of such a spell is inherently the liar, but you don't know if he is or isn't a liar, or what he is or isn't lying about, before you cast the spell.
I'm not sure if I'm explaining this clearly, but it's the best I can do for right now.