Sure. Magic that forces anyone's character to do anything they don't choose to do (overt mind control, blood boiling in their veins, transmuting their flesh into potassium ferrocyanide in strong acid to produce hydrogen cyanide gas, etc.,) is explicitly not allowed. Anything that causes a single character to be able to lay waste to an enemy army? Not allowed. Anything that forces you to use your powers creatively in order to maximize your potential damage output should be encouraged (ie, a single fireball cast upon the surface of an ancient, rickety, near rotten wooden staircase an enemy force of, say, forty soldiers is running up in order to get to you might do more damage than a fireball forty times the size of the original example would to half the number of soldiers on an open field) and, most importantly, if someone's abusing their powers, tell them to knock it off. It's your project, and if you feel someone's overstepping their bounds do something about it. 

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Additionally, I love your avatar. Just sayin'. <3 

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Also, asking for a detailed explanation of a the depth of a mage's powers in their character sheet can stop most problems from occurring in the first place. If you come across a character whose list of powers includes anything dubious or outright overpowered it's best not to allow them without edits, and should someone start using powers that aren't listed in their character sheet (or, at least aren't within the spirit of the character's abilities. A mage who can throw a fireball can probably light a cigarette with a smaller, more controlled flame, but if said spell is the strongest thing they've got listed it's unlikely they can rain meteors down upon their foes) it's easy enough to remind them that they themselves chose not to have such a breadth of sheer damage output.

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Your name's wonderful, as well. Chemistry jokes need to happen more.