Browser extensions are probably the most convenient option, especially for the layman, but there is always DNS-based blocking and/or local proxies like privoxy.
Well I can’t know what you’re talking about precisely because you didn’t give any examples, but my sense is that people involved in things like Wikipedia are going to lean (American) left because Wikipedia is socialism and it embraces progress. Most of the rest of the world is left of the American left and Wikipedia is also international, so that would exacerbate things to some degree.