If you think about it, the Q aren't nearly a threat to the galaxy that the Borg are. The Borg are out to get everyone, while if a Q goes around bugging, teasing, or otherwise harassing various species, then the Q Continuum might punish them by, say, turning them into a human and sending them naked to their favorite star ship. (Of course there were repercussions to reality during the Q Civil War).
Who knows how long Guinan's specie knew of the Q. Perhaps long enough to create a superstition as how to deal with them?