In general there are 3 solutions involving 'moderation':
1) self moderation: Google Plus has circles which you can theoretically use to share specific topics only with circles that you know to be passionate about them. Requires manual work to setup the right circles and people rarely do it.
2) manual moderation: Hacker News uses pg's time and some admin/moderator supervision to trim unwanted topics. Requires manual work which expands proportionally with the community's size if no automation is used.
3) automatic moderation: HN's flag system and community moderation (voting, Slashdot karma) seem to work relatively well, but they've been used until now predominantly to "rank" good comments to the top as opposed to "clustering" online communities into "sub-reddits" with focused interest groups.