There is an easy way and a hard way to do this, depending on what you prefer.

Hard way: Any of the above suggestions, particularly creating new classes. That is the best way to get EXACTLY what you want, though it will take considerable work. Isolating types of spells is also a good idea, but ultimately a level 7 is a level 7, no matter how you cut it.

Easy way: This doesn't work for all flavors, but it's a nice idea and, most importantly, relatively painless. Remove Clerics, Druids, Sorcerers, and Wizards from your lineup. Any class, in fact, that goes up to 9 level spells, or is a primary caster. Get rid of them all. Now you're left with Rangers, Paladins, and Bards. Bards get spells up to level 9, but most of their stuff is pretty 'safe' - illusions, enchantments, etc. No real big destructive blights on the land. But, hey, you can tone them down, too, if you want. Or remove them outright (you don't lose much, though you will now have no arcane casting class).

Your end effect here is that Rangers overtake the Druid role, Paladins overtake the Cleric role, and Bards overtake the Wizard/Sorcerer role. Certainly, you no longer have the cloistered archaic merlin type, but that's just a sacrifice for the 'easy' method.

Alternatively, you can add Wizard/Sorcerer/Druid/Cleric back in as Prestige classes, rearranging their class benefits and spell progression, limiting it however you see fit. I guess that would be my 'intermediate' suggestion between 'Easy' and 'Hard'. But mostly, I like the Easy way.

Too bad it's not always flavor appropriate. :-/

EDIT: Oh yeah, and CRs are going to be inaccurate with low magic.