Well actually Reimu's "Here in Gensokyo, those who quit being human, Hakurei must destroy" . only canonically applies to willingly with darkly and forbidden methods, Balance-Threatening Youkai-Turned Human Villagers ,or at least still significantly tied to the Human Village and affecting the Human -Youkai balance.
The only case is the the "Jinyou/Youkai-Turned Human Fortune Teller" from the manga FS/Forbidden Scrollery who also was showing these signs first by his planned and commited Suicide ,a commoting-bearing one inside the Human Village while still a Human Fortune-Teller ; then possessing and exploiting Kosuzu the Suzuunan Library while later causing a lesser ruckus ; his outer appearance a clear indicator in the touhou setting ; he was trying to fully justifying himself before Reimu;f he was finally and mostly he was"Truly Exterminated by a single True Hakurei Sealing Talisman" but since those are the More Exorcising and Purifying the More a Youkai or is Evil and Tainted that confirms he was an highly Evil/Tainted Youkai... his opposite would be the "Young(Youkai) Fox Student" (in the samr FS/Forbidden Scrollery printwork) Innocent and Pure to the point he opened its way without damage or notice through an "Hakurei Talisman Seal Blocking Panel( Dozens of true Hakurei Sealing Talismans entwined)" ...
A case such as Marisa or similar enough ones would not be bound,unless they allow an irreversible decay to themselves (again ref. the "Jinyou/Youkai-Turned Human Fortune-Teller" from FS/Forbidden Scrollery) , since no longer a Human Villager,mainly secondary ties and interactions with the Human Village,Youkai Magicianhood not achieved through dark ,forbidden Human-Youkai Balance-Threat means.
On an final half-related note. Although they are not Youkai-turned humans the end result would still be enough compatibile for eventual such ones, the "Grassroot Youkai Network" is a good starting point and a member,Sekibanki, is even a good,living,also almost-hidden,Youkai-Blended-Balanced Resident at the Human Village.
