LG, NG, CG: Accept the giant's surrender. Disarm and unequip the giant. Blindfold, bind his arms behind his back, and connect that rope to bonds at his ankles to limit his stride. Inform him that he is a prisoner, and that any attempt at escape, communication with his allies, or attacks on his captors will mean his immediate execution. Take giant back to camp, where you interrogate him thoughly on any and all information concerning the enemy, using truth-verifying spells if possible. Mind read if neccessary. After interrogation, use him as a bargaining chip with the enemy side for prisoner exchange ... assuming they've taken any prisoners. Keep the giant as unaware of information on your side as possible, possibly including feeding him false information. If they other side doesn't take prisoners, release back to his allies to relay the false information you've fed him, and to inform his supperiors that no further surrender will be accepted.
Alternatly, if said giants have shown themselves to be without mercy or compassion - not taking prisoners, raping, killing non combatants, etc. You could just kill the surrendering giant. They gave no quarter to your people's cries of succor, you are then under no obligation moral or otherwise to accept theirs.