Can I lower a personality's force to zero with The Direct Approach to prevent that personality from assigning as an attacker or defender for the turn?
The Direct Approach and assigning?
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
No.
If the targeted Personality's Force is 0 when the last effect of The Direct Approach applies, he may not perform actions. However to assign one's Personality normally during the Maneuvers Segments of an Attack Phase is not an action. An action has a designator: Limited, Open, Battle or Reaction. For more information, see Card features and Abilities and actions in the comprehensive rules.