Actually, it seems that the Haitian's amnesia ability works on normals (HRG and HRG's normal family members, certainly), too, so I suspect his ability to render somebody unconscious does as well. In fact, I suspect his power nullification actually works by making a special momentarily forget how to consciously access his or her powers.

Still, it makes sense that Buffy would be able to resist him even so, given her exposure to Dracula and other mindbenders in the past.