Or say "Assume I just rolled the utterly pointless roll that counts for absolutely nothing and will be ignored. K? Right, here's the roll that will count." Then roll the dice.
CoDoom, why on earth would anyone bother rolling a roll that is always going to ignored, whether it is good, bad or indifferent? You could, to exactly the same effect roll the dice under a box, never look at them, then pick them up and do the re-roll.
Just state that this is to be your 'second' roll, ands roll once. If anyone argued that was cheating, then you're playing a mad-man - smile, keep your guard up (but don't show it) and back slowly out of the room.