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.
A player, 8 January 2014
Main question: can Hantei Sensei still prevent Kharmic Strike's effect?
Waaay back when, there was a ruling that KS, "If named by Hantei Sensei, it is considered to have a blank text box. [JA, 7 July 2000]"
From Hantei Sensei's Oracle text, that is not immediate to me (does "play a card" extend to Focus Effects?).
Additional related-but-not-really-rules question: what (other) card effects in the L5R card pool can prevent Kharmic Strike's effect? I've come up with Shattered Focus (per a previous rules question) and Mirumoto's "Niten"...
A player, 8 January 2014
It has rained a lot since 2000! The new Oracle of the Void wording of Hantei Sensei is slightly different and therefore the old ruling is no longer safe.
Limited: Once per game, target another player: You, then the other player, each name a non-Ring Fate card. Until the game ends, that player may not play any copy of the card you named, and you may not play any copy of the card that player named.
Under current rules, "play" means: As a verb, “play” can refer to: putting into play a card type that normally enters play; using a Strategy card by putting it into the resolution area; bringing a Ring into play; or using an ability on a Ring that is out of play.
Focusing a card and resolving its focus effects do not count as any of those.