If it's a pub that I know, or know about, it can indeed be the people in a pub that put me off going there. However I occasionally have to research unknown pubs for lunch with a veggie friend when visiting a town that I don't know well. Possibilities tend to get crossed straight off my list if they've only got a Facebook page, rather than a proper website.
Trying to track down a current menu, and to see how well the pub caters for vegetarians, is usually impossible on a Facebook page but dead easy on a website. (There are even pubs, with only a Facebook page, who set it up so that one has to be logged into Facebook in order to view it, thus losing potential trade from people who simply don't 'do' Facebook at all).
Away from that though, other things that might put me off venturing into a strange pub are massive signs about their multiple big screen TVs, with listings of all forthcoming big matches, and similarly big signs declaring the venue to be a 'sports bar'. The dreaded word 'karaoke', seen on a sign outside a pub, tends not to tempt me in either!