I don't know. If you delibrately pull his magic to hit you, then I am not sure he has tried to kill you and therefore committed a crime. He only forfeits immunity if he commits a crime, so you cannot attack to kill until then. But you are perfectly entitled to intefere with the duel by using magic to block spells completely with perdo vim effects.
You would be open to a charge from the enemy mage's allies if he dies that you killed him as you had deprived him of magic while he was in a fight for his life, but if you do not cast any spells that harm him you should get off. Especially as you are legitamately defending yourself against becoming collateral damage which if he had hit you then he would have broken the law as a high crime, as you have not forfeited immunity yourself. He is also not able to do any attacks which damage your covenant without breaking deprivation of power as a high crime if he damages magical equipment or the library or a low crime if he might hit the infrastructure or staff, so you can block spells with the reason of preventing him from committing crimes.
You are not even open to a charge of depriving him of magic, as he is perfectly capable of casting the same spell again the next round, depriving of power is about reducing his magical ability in the short term at least. As always these open to interpretation cases, always depend on your relation with the tribunal and whether you are likely to win any minor vote against you.
And it's always easier to beg forgivness than ask permission.
thrakhath makes some great points, and he's right that whatever charges are brought against you (and that stick) are largely a matter of your relationship in the Tribunal.
However, if I were a Traditionlist Quaesitor, I would look at the Code, and say that you both deprived the "poor" magus of his magical power with a Perdo Vim spell and that you "slayed a member of the Order outside of a justly executed and formally declared Wizard's War".
The Peripheral Code says that damaging a lab is equivelent to depriving a magus of his magical power, so I would argue that destroying a cast spell might fall in the same light. Lets say you tried this multiple times, I might say "Not once, not twice, but THRICE did this magus destroy the power of the deceased!" Admittidly, I might be streaching things a bit here, but multiple charges often increase the chance that something will stick.
The tougher charge to beat would be the murder charge. No reasonable magus would deny your right to defend yourself, but killing had better be the only way to have stopped this mage, AND you better be able to prove he was going to kill you. Even then, there better be some even minded Mages at the Tribunal. The Code doesn't seem to allow for immediate justice. Someone who breaks the Code (as in attempting to slay a member of the Order) requests in his Oath that he be cast out. IF he is cast out, he is to be slain.
My point was that depriving a fellow wizard of power is written in the rules as you should not interfere with a fellow wizards ability to use, practice or study magic. Practicing and using being in this case the same, we are talking about the ability to use it, not the use of it itself. By cancelling one or two spells you have not reduced his ability with it at all. All the examples listed were in order of badness from least to worst: killing servants, breaking the covenant, damaging labs or libraries, killing apprentices or familiars or damaging vis sources or magic items, physically disabling the wizard, burning his gift out. These are all long term factors and the weakest of them are considered only low crimes. To prevent a spell, an act that has no long term effect, would in itself be a low crime at worst, especially if you had good cause.
My other point is that you cannot be charged with slaying a member of the order if you do not cast any spells which affect him at all. If all you do is cancel some of his spells that you think are overkill and would hit innocent bystanders or damage your lab (crimes that would have caused him to forfeit immunity), then you have not harmed the other wizard. Nor would you if you cancelled every single spell he casts and allowed your sodales to cast spells back unopposed. As far as I am concerned slaying another wizard is casting spells that hurt him while others finish the job or that kill him outright
"Cannot be charged" is hardly true. Even in the most law and order saga, a Mage can easily be charged with almost anything, since it only takes a single wizard to bring charges to a Tribunal. Doing it effectively is certainly much harder.
Your opinion is certainly just, however justice isn't the only factor in any punishment.
Huh? Thats one heck of a loophole. So if I just brutaly stab my sodalis to death with a non-magical knife I cannot be charged with slaying in your opinion?
Intrestingly, again, reading that section of the code, I came apon something that seems to refute this.
"The prohibition against retribution is interpreted as complete legal immunity from charges for the slaying. It also [EMO] forbids other magi from persecuting the victor for the slaying."
OK, so it is clear that retribution IS charging the magi for the slaying, but the passage specificly seperates persecution from retribution. So it comes down to what exactly does persecution entail, legaly speaking?
Sorry Degamer, I should have spelt out in clear obvious terminology that I meant any attack that hurts or kills rather than just spells. If you had read the previous discussion you would have seen this was regarding the use of spells countering magic during a wizards war between two other people. I maintained that not performing any offensive action towards the caster meant you were not guilty of attempting to slay him.
I just wasn't sure how many people still recall Fred MacMurray - despite his great performance alongside Stanwyck and Robinson in that movie. And then Mitchum's fall guys are more heroic and sympathetic, of course.
While some of us may not be COOFs ,
one is old enough to remember watching first run episodes of Dr Who with William Hartnell.
So Fred , Bob and Babs are definitely in with a chance.