[joeysteele]

They would probably threaten to sue them or something daft for breaking their contract and not abiding by the rules of X Factor.

It would be a good thing to see happen though.
I am trying to imagine the judges faces if it ever did.
Having said that, if a judge can abstain from voting an act out then an act should be able to abstain from taking part in a sing off, especially if they know as the opening post said, that they are going to be put out anyway, no matter what their performance may be.