If everyone's overpowered, then no one's overpowered. (i.e. every single PC being OP is actually pretty balanced)

Add some magical laws. either those described earlier, or ones like "obliterating stuff means you get obliterated" or things like that.

Another option is to make them balanced with another opposite power, where they can negate each other. (fire/water, "earth/air", Life/death, "Healing/sickness", etc.)
Or you could add side effects. like: burn something, you get a fever. Freeze something, you get chilled. hurt something, you feel the pain. if anything, that'll limit most magi easily. of course, if that doesn't work and they still make their magic too powerful? Kill the PC.
If the RP isn't deadly serious... Make at least parts of magic a joke.