I think this is part of the "A wizard can do anything with enough preparation" thing. Judging from the books, shutting down a wizards power is a full time job. Harry does this to Kravos, Ebenezar does this to Mavra, both have to keep their full concentration on the other to make it work.
I think a good way to do this would be a thaumaturgy maneuver placed on the target before going into the scene (that can, of course, be resisted), and then tagging it to go into a discipline grapple (affecting magic in this case, not physical actions). Both the blocking wizard and the blocked wizard will be unable to cast any other spells, as long as the grapple is kept up, but if the blocked wizard can break free, you can't regrapple him, because the spell is broken.
It is not too powerful, I think, most wizards will have similar skill levels there. It is also not too weak, because it gets the job done. It is not too boring, because it keeps the struggle for power up throughout the scene, instead of shutting the wizard down with one spell.
Or you can go, of course with a simple thaumaturgy block, which will still get the job done, but I find rather boring. Plus, it has a fixed number of shifts, and is vulnerable against a good roll and some tags, while the above can be modified as need, too.
I don't really like the mental attack thing though, it seems much more drastic somehow.