How do you do that? The lawyer's conduct was geared towards muddling the decision over whether to execute or not. I agree that in an ideal world the lawyer's conduct should have no bearing upon this, but in reality lawyers are not always totally professional in their conduct and rules are made to discourage their treachery. In this case, the lawyer got caught and it may be the duty of the judge to throw out the case.
Without safeguards against corruption on the part of prosecutors, the ability of the innocent to defend themselves is significantly weakened. Should I ever find myself wrongly accused of a crime, I want all of the protections that I can get - even if it means that that terrorist murderers serve life in prison instead of getting the chair.
In any case, when the rules are established in advance, it's tough to fault anyone other than the participants when the system produces undesireable results.
You do that by getting a new jury, letting it decide the perp's fate after a new penalty trial, and by punishing the lawyer separately. I repeat--a criminal trial should be about the facts of the case.
Any illegally obtained evidence or prosecutorial misconduct happens AFTER the crime. If the accused did the crime and the facts prove it, then he needs to be found guilty. Any other outcome is unjust.