You could try playing a difficult clan in an Expert Sengoku campaign and forbid yourself geishas. Ninjas become the only way to get rid of the nasty enemy kunoichis which eventually appear, aside from destroying the Geisha House where they are being trained.

Build as many as can be afforded, as early as possible, and level them up by training them against emissaries, priests, and one- or two-star generals. As Sasaki mentioned, you can train +3 ninja with Geisha Houses in either Yamashiro or Iga. I use Iga; Yamashiro is the only province capable of training +3 Yari Samurai and I tend to focus any given troop-training province on one troop type (producing one per turn whenever possible). This is necessary because I tend to employ a constant and relatively rapid expansion/conquest strategy, which requires a regular flow of new troops.

Or, just try to win before an enemy clan is able to produce the hairpin-throwing biznatches.

All in good fun.