Nullification is balanced around the hung jury process. http://en.wikipedia.org/wiki/Hung_jury If a jury can't decide in some time frame the judge can decide to end the trial without a verdict and enable a new trial to occur. Your constitutional protection vs multiple tryals only occurs after a jury has said not guilty it does not apply when a jury can't decide on a verdict.
PS: In the US In the United States, the result is a mistrial, and the case may be retried. Some jurisdictions permit the court to give the jury a so-called Allen charge, inviting the dissenting jurors to re-examine their opinions, as a last ditch effort to prevent the jury from hanging. The Federal Rules of Criminal Procedure state, "The verdict must be unanimous...If there are multiple defendants, the jury may return a verdict at any time during its deliberations as to any defendant about whom it has agreed...If the jury cannot agree on all counts as to any defendant, the jury may return a verdict on those counts on which it has agreed...If the jury cannot agree on a verdict on one or more counts, the court may declare a mistrial on those counts. A hung jury does not imply either the defendant's guilt or innocence. The government may retry any defendant on any count on which the jury could not agree."[7]
PS: In the US In the United States, the result is a mistrial, and the case may be retried. Some jurisdictions permit the court to give the jury a so-called Allen charge, inviting the dissenting jurors to re-examine their opinions, as a last ditch effort to prevent the jury from hanging. The Federal Rules of Criminal Procedure state, "The verdict must be unanimous...If there are multiple defendants, the jury may return a verdict at any time during its deliberations as to any defendant about whom it has agreed...If the jury cannot agree on all counts as to any defendant, the jury may return a verdict on those counts on which it has agreed...If the jury cannot agree on a verdict on one or more counts, the court may declare a mistrial on those counts. A hung jury does not imply either the defendant's guilt or innocence. The government may retry any defendant on any count on which the jury could not agree."[7]