I remember, reading a Dom/Fem Sub/male Spuffy story that takes place, after Dead Things. Where Buffy gives into the dark side of the Slayer and she finds a way to control Spike with a collar or a ring or something. She then proceeds to dominate Spike completely, with BDSM, by making him her pet vampire.
Over time she allows her real feelings to come out, for Spike and she doesn't abuse him, physically and emotionally anymore because she feels in complete control of their relationship, at the end of it.
There is a chance it could be after S5 Crush but just not sure of it, now. | 
