depends on the platform. on twitter i tend to block very liberally for reasons ranging from 1. having a bad take that mildly annoyed me 2. did something mean/bad to one of my friends 3. varying degrees of bigotry, 4. mean/rude to me (either directly or tweeting something that implied you WOULD be rude to me), 5. you draw/post about my notps and i don't know you (if someone i know does that i tend to mute rather than block), 6. you're a minor who didn't listen to the all caps no minors in my bio. on AO3, really the only reasons i block someone is if they've done something to hurt me or if they put unsolicited concrit in my comments and/or a rating/unsolicited concrit on one of my fics.