(BNW)Granted, I understand it's exceptionally difficult to adopt every social policy and ideological tenant before achieving any other victory condition... but I actually managed to do it by accident and kept a game running solely for meta-purposes. Technically speaking, one could force a world ideology through the world congress and adopt all new tenants for both of the ideologies they didn't choose, but even after that, there appears to be no recurring way of spending amassed culture, unless I've overlooked something completely and would feel like a total moron
As far as I know, there's nothing to do with excess culture. In Brave New World, at least, having massive Culture reserves would prevent a cultural victory by some other Civ, but there's no other use for it.
Prior to BNW, once I had got too much culture I would jump from the Autocracy / Order / Freedom trees.