There's no such thing as "partially using" a power.
That said, all damage in 4e can explicitly be non-lethal, at the attacker's discretion. From the PHB pg. 295:
When you reduce a creature to 0 hit points or fewer, you can choose to knock it unconscious rather than kill it. Until it regains hit points, the creature is unconscious but not dying. Any healing makes the creature conscious.
If the creature doesn’t receive any healing, it is restored to 1 hit point and becomes conscious after a short rest.
This could be utilized to torture any creature (undead or otherwise) using any power, but isn't always efficient, as every time you knock something unconscious it will take it 5 minutes to recover unless you have/are willing to use a healing power on it that neither costs a healing surge nor restricts targeting to "allies".
So the answer is no, the cleric cannot intentionally reduce the amount of damage done in any way, or intentionally roll minimum (and the dice portion of the damage will lose much of its meaning by mid-paragon anyway), however the cleric can choose not to kill the target with the damage dealt, which can be fluffed however you/he want it to be fluffed, as fluff is explicitly entirely discretionary in 4e.