Coud it be 'Ring Around The Buffy'
The Story: Buffy and Spike go cross country and under cover to infiltrate a vampire human slavery ring.
Rating: PG — NC17.
Disclaimer: None of us own any of this, Joss is King.
Spoilers: General Season 5.
Written By (in order of appearance): Annamarie, tanith, VicNoir, Kristin (aka Love's Bitch), Blaque Kat, Spikelicious, Tracy (AKA Jericho TGF), Jessa, NautiBitz, Raven, Saber ShadowKitten
I haven't read it (yet) but looking into it, there's the undercover part, the cages and the spanking! Anaunthe wrote a conclusion
Hope this is the one, keep us posted!