Enforce mana pools and tie it to the amount of energy exerted or matter produced. Coupled with detailed explanations of the magic's workings, you will be able to easily regulate a characters power.

They throw a molten bolt of tungsten, heated to 5500 celcius, at mach 4 speeds? Okay, but producing the metal, which is pretty dense, heating and accelerating it will take a damned lot of mana to do. Casting a napalm fireball? Okay, it has less damaging potential, but napalm is lighter and needs lower temperatures to ignite, so the spell is overall cheaper.

Also, be strict with the diversity and range of spells: the more they have in their arsenal, the less proficient they must be with each spell, and vice versa. Including such a thing as "schools of magic" may also help, forcing the player to wisely choose, rather than just take everything they want and throw off the roleplay's balance.