There isn't an absolute reputation score. Reputation is computed pairwise. Rep(a,b) means "What is the implied reputation of person b, given the persons a trusts"

So a bunch of fake accounts aren't going to influence a's perception of b,unless a has trusted someone who'se created those fake accounts. IN that case, the mistake a made was trusting someone (directly or indirectly) who'se created a bunch of fake accounts.

Because the input weight of a person decreases with their distance on the graph from a, the only sybil attacks that would screw you up would be ones conducted by people who you've said you trust.