After reading the threads on spells to encase an opponent in something, and spells to increase MR, I started wondering if it would be possible to create a spell to encase an opponent with an "inverse parma". The spell would let the target cast R:personal spells without problem, but casting a spell with a greater range would require the caster to overcome the encasing "parma".
The other encasement spells would be CrFo, but this seems like it would be a PeVi spell:
Containing the Magic
R: Voice, D: Concentration, T: Individual
Spells cast by the target at greater than R: Personal are destroyed if their level is less than half the level of the Vim spell. Thus, a level 20 CtM would stop spells of level 10 or less, and require a casting total of 35 (Base, +2 Voice, +1 Concentration).
Alternatively, this could be a Hermetic Breakthrough, along the lines of learning how to extend your PM to another, but instead extend an inversion of your PM to another.
This reminds me of a joke.
A professor of Literature, a student of Physics and a student of Maths have a contest who can build the smallest sheep fold to enclose thirty sheep.
The professor of Literature decides to build a circular sheepfold.
The professor of Physics builds a semicircle - the other side is made up by the farmhouse.
The professor of Maths wraps the fence around himself and says "I define myself as outside".
Well, there is a PeVi guideline to reduce the Casting Total of all magic cast by a target; not quite what you are describing, but a similar effect : instead of needing to Penetrate 20 MR, he needs +20 on his Casting Total... I guess you could even add a Rego requisite to let him cast Personal spell unimpaired if you really wanted to.
It seems plausible and would be darn high base effect level, but as I started thinking about how to construct it something else came up:
Just to take the range at which the effect is used out of the discussion for a moment - isn't this more a Rego effect which is essentially stopping magic from passing through, rather than destroying magic? A Perdo effect has nothing to actually target if a spell is not in the area just around the targeted magus. It feels very much more like an aegis effect, and I'd be tempted to make the base effect the creator is looking for moderate, but have a few mags added for the complexity. There is also the note in the Rego Vim guidelines that indicates that Parma Magica was not able to be created as a spell, and this seems a good consideration when looking at an effect which is essentially very similar with a different target.
I'd say its valid to use a Perdo effect if this was a linked effect in a magic item. One spell detects the magic, the other destroys it. Even then I'm not very comfortable with that idea as I think you need a fast cast effect to ensure you can target a spell that is "in-flight"; and items can't fast cast (correct?).
A second thing to consider is that the targeted area could easily be circumvented by a personal teleport effect (ReCo10, a 5 pace teleport) that is not too hard to spont, and it is certainly a direct attack.