How does "unmaking" happen? What power does Buffy have to "unmake" the Slayer line?

Well, she "made" them through magic, so "unmaking" also could proceed through some spell and shiny mojo.

Fray shows that Buffy does not unmake the line, because she (Fray) was the first to be called in centuries because the monsters returned. Using this information, we can amend the verb "unmake" to "postpone," and "assume" that Buffy et al. kill off all the vampires.

Good point. There even may be some twist: for example, slayers lose their power as soon as the last vampire is dusted (soulless vampires, that is).

Kind of universal balance. :)