Let me preface by saying I am not a lawyer, but from what I read and what I have followed over the years:

Afik you cannot reject commutation: commutation happens when one punishment is replaced with another, less severe one... I.e. people on death row being taken off and put into life without the possibility of parole. I know in Connecticut, when the governor took everyone off death row, some of the inmates sued him in an attempt to keep it from happening, but I don't think they were successful.

On the other hand: When someone is pardoned, their conviction stands, it is not removed. They are still a convicted felon, but their legal consequences are removed, they also are not able to be prosecuted in any way again for the same crime/set of crimes. If they are pre-pardoned (i.e. before a judgment is made) the case is withdrawn and does not move forward. It has little do to with conviction status, more to do with the punishment phase, and since there can be no legal consequences, there is no point in pursuing the trial.

Ultimately I think (especially in his guys case) it's moot. His reasons to reject the pardon is he realizes what he did is felonious, and the pardon doesn't change his felony conviction status.