It really bores me when someone talks about one specific thing all the time, nonstop, but the topics that are boring to me are anime, anything related to kpop, politics, clothing/fashion/makeup or sports. And when girls talk about guys (or boyfriends) all the time... that bores me and actually annoys me. Nothing wrong with talking about how cute someone is but chill, there's other things to talk about.
I usually just stop talking to people I brand boring after a conversation and avoid them. I feel rude if someone is rambling on and on about something I have no interest in, but is important to them, and I ignore them/don't appear interested.