Ah, but Urdinaran...what exactly do /you/ mean by the "spirit of the game"? Is using the Defensive Blast ability very simply as written a violation of the "spirit of the game", then? I always thought I knew what "spirit of the game" meant until now, and had assumed that violating the "spirit of the game" involved one of two elements:
A) Bad roleplaying. This can, for example, involve a character using out of character knowledge, or behaving in a way that does not mesh with his stats and abilities, such as a low intelligence person coming up with brilliant plans. Or in a stricter sense, though it's difficult to tell from the outside, behaving contrary to his own character concept because he finds it expedient to do so at the time.
B) Twisting the rules to have unbalancing effects beyond the obvious effects as written.
Neither of those two are involved, are they?
The rules for Defensive Blast, as written, without ANY clever manipulation, rules lawyering, or trickery WHATSOEVER, allow a sorceror to blow up pretty much anything, almost at will.
There are, in my opinion, only two viable ways of dealing with this: suffer it, or change the rules.
Otherwise, hmmm...take this theoretically common scenario:
The sorceror, knowing he has defensive blast, finds himself in a fight in closed terrain, such as in a building. He does not run away from the enemy, though one might expect a weedy sorceror to run away from harm. The enemy, predictably, close with him. Bang. GM calls him a munchkin. Player, who is quite principled and wouldn't dream of committing either A) or B) is rather hurt by this accusation and asks why the hell the GM doesn't either fix the rules, explain in each case when and how the abilitiy is viable at the time in a consistent fashion, or just run his character for him. And everyone ends up old and bitter.
It's not the only rule that's broken (see the Rule of the Master, which allows any sorceror to siphon power points at infinite range from any of his sorcerors thralls) and hopefully with a second edition they will have fixed them all.
Meanwhile, that's what this board is for. I like Cavalorn's suggestion best; it's very well thought out. He's limited the ability as well as actually making it a bit more defensively useful by being able to trigger it when one is dangerously wounded, outside one's own actions.