I believe it was in the 1920s when the issue came up, but basically the Supreme Court ruled the purpose of a Pardon is for the general good, thus even if the Defendant does NOT want to be pardon, the pardon is still valid. The case involved the commutation of a Death Sentence to one of Life imprisonment. After 20 years in prison the Prisoner filed a Writ of Habeas Corpus on the grounds he had never AGREED to the reprieve. The prisoner maintained that without his consent the method of punishment could NOT be changed and thus the reprieved was invalid, the court took a different view of what a Pardon was:
We will not go into history, but we will say a word about the principles of pardons in the law of the United States. A pardon in our days is not a private act of grace from an individual happening to possess power. It is a part of the Constitutional scheme. When granted it is the determination of the ultimate authority that the public welfare will be better served by inflicting less than what the judgment fixed. See Ex parte Grossman, 267 U.S. 87, 120 , 121 S., 45 S. Ct. 332, 38 A. L. R. 131. Just as the original punishment would be imposed without regard to the prisoner's consent and in the teeth of his will, whether he liked it or not, the public welfare, not his consent determines what shall be done. So far as a pardon legitimately cuts down a penalty it affects the judgment imposing it. No one doubts that a reduction of the term of an imprisonment or the amount of a fine would limit the sentence effectively on the one side and on the other would leave the reduced term or fine valid and to be enforced, and that the convict's consent is not required. http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=us&vol=274&invol=480