Here're some AH fics where they are either enemies, cop v criminal, or extremely antagonistic towards each other:
Common Knowledge by Panta_Rei
Summary: Everyone knows that Buffy Summers is a pretty, popular cheerleader with a perfect life, just like everyone knows that Spike Walsh is a dark, disturbed punk who's more arrogant jerk than decent person. Everyone also knows that they hate each other. So what do you do when it turns out that what everyone knows is just plain wrong?
Criminal Games by Ashlee
Summary: William 'Spike' Reed is the most successful art thief on record- and he's never been caught. As one of the most powerful men in the country, he has managed to elude arrest for years while still maintaining a prominent status. When FBI Agent Buffy Summers is assigned to go undercover to capture him, she's entered a game that she's not sure she can win.
It's Okay; I'm With the Band By Starmouse
(Fantasy Fic. Two members of the punk band XCOD have landed themselves in extended detox, leaving bandmates Xander, Oz, and Spike scrambling to fill in the blanks before the tour starts. Permanent WIP but ends in an interesting place IMO.
Mixing Business with Fanfic by Sotia
Summary: In a Universe where BTVS never existed as a show (*shudders at the thought*) Elizabeth - Beth - Springs has just gotten a job with her step dad's company. On slow days - which come about a lot of the time - she reads Spuffy fanfic that seems only accessible from her work PC. What happens when William, head of the IT department and the only person seeming less than happy with her working there, decides to find out what she does in front of her computer screen all day | 
