So I'm tryin to get an official response here, from ffg in particular, since I can't seem to find a direct answer from them, but does using councilman to derez a sansan stop the score? I believe it should but I would like a direct answer before going into regionals.

FFG don't answer in the forums. If you want an official response, ask in the rules question link below.

However, the Chain Reaction rules found in the FAQ mean that yes, unlike Political Operative, Councilman can prevent scoring form SanSan City Grid. This is because even though rezzing SanSan and scoring happen in the same window, Councilman is triggered (and so must be resolved) when SanSan is rezzed, before the Corp has a chance to score the agenda.

By contrast, Political Operative must wait until the Runner has priority to act in the paid ability window, by which time the Corp can both rez SanSan and score the agenda before passing priority. (I know you didn't ask about Political Operative, I'm just trying to forestall anyone else asking about it in the same thread).

Hope that helps.