... no, Ramza's making perfect sense, though possibly not too clear. Here's an example:
Edit abilities and select "Devotion Aura". Where it says "Level 1 - Data - Armor Bonus" hold down the shift key and double-click that value. Since you held down shift, you can type in negative numbers.
When you pick Devotion Aura in the game, it will now reduce armor instead of improve armor. Try it with other auras, though it may not work for all of them. I just tested it on the one mentioned here.