You basically want to put a sticky aspect on a zone that you immediately tag for free to put up a block on magic. You don't have to get too into the thing with running water shorting out magic here to do it. You could always describe it as an entropy effect on that area targeting magic only. The only potential problem I see for this spell is it'd be kind of hefty. Looks like it would either not be all that strong of a block or it would cost you a lot to get it off. And all you really do is set a minimum target number for any spellcasting roll in that zone. You obviously beat the block if your Discipline roll has a higher difficulty just to get the spell off than the block rating.

It is a very specific feature of blocks, and magic blocks in particular, that they dissapate as soon as something beats one.  In fact, one of the reasons the developers made armor harder to do than blocks was because armor doesn't dissapate from being overcome, while blocks do.

And to expand on example 3, when you cast a spell, you are testing against several different values with a single Discipline roll.  First there's the shifts of power allocated to the spell; if you don't meet or exceed this number the spell will cause backlash/fallout but might still affect the target (albeit as a potentially weaker spell).  In addition to this you need to meet or exceed the difficulty of the spell, which is each a fixed difficulty determined by the GM or a defense roll made by the target.  A Block, as described above, affects that second test only.  This is true whether the Block was a defensive spell cast on the target or an 'offensive' spell cast on the attacker to prevent an action.