Like I keep saying, there's likely some other law or effect in place when it comes to youkai that are sealed away. Suika specifically states in Immaterial and Missing Power (or... somewhere) that she's only going to be in Gensokyo temporarily, and it's also stated in her ending (...or somewhere) that one of the (possible?) reasons she was unable to bring the oni up to Gensokyo to follow her was due to her not sincerely attacking humans (a personality quirk that made her a bit abnormal compared to other oni).
I don't really think there is a law that stops Suika from hanging above ground. We know Suika is above ground during Immaterial and Missing Power and she is probably above ground all that time till at least Subterranean Animism. We just don't know where Suika is hanging out. We also know she appears in several pages of the 3 fairies manga. I think in Wild and Horned Hermit 8 and 9, we again, find out she is above ground.
From Suika's conversation with Yuugi and pretty much many of the SA conversations, it appears that Suika hasn't been underground for a while.
Maybe rejection works, I can't really tell, since I don't see any examples. I am also unsure if youkai like Kyouko actually disappear when that happens. It seems like they just lose their job or way of life so to speak.
@Sophilia: But Patchouli may not know what Oni are. When she met Suika on her route, she knew what Oni are. However, when Suika meets her on Suika's route, Patchouli has no idea what Oni are. In either case, she had to do research on Oni afterwards. We see this presented in the manga when the Scarlet Devil Mansion had a Setsubun festival and find out that it also burns Remilia.
