The Player's Handbook says in the section about Damage
When your attack succeeds, you deal damage. The type of weapon used (see Table 7–5: Weapons, page 116) determines the amount of damage you deal. Effects that modify weapon damage apply to unarmed strikes and the natural physical attack forms of creatures. Damage reduces a target’s current hit points.
Minimum Damage: If penalties reduce the damage result to less than 1, a hit still deals 1 point of damage.
Strength Bonus: When you hit with a melee or thrown weapon, including a sling, add your Strength modifier to the damage result. A Strength penalty, but not a bonus, applies on attacks made with a bow that is not a composite bow. (PH 134)
I know a creature can take a -4 penalty to inflict nonlethal damage with a weapon that inflicts damage, but is there a rule that permits a creature to voluntarily reduce the amount of weapon damage he inflicts? For example, can a creature choose to employ less than its full Strength when making an attack?