it varies on the person, tbh. some of them you have to list multiple names because they go through them like underwear, either to avoid bans or for continuing to figure out who they want to be online. some of them are serious problems in games because of their behavior, their inability to communicate, their boundary pushing/ignoring preferences. serious things that make the game miserable. they fight the mods. they manufacture wank/doctor screencaps/sic people after bad cr. they're bullies. then there's the moderate/medium annoyances that ask too many minutia questions, constantly pester the mods, don't listen/don't read properly, constantly have communication issues because they don't talk to people. they're the kind of player that makes a mod groan when they open their email and see a notif from them. they make the game less fun to run and play in.

if a mod doesn't want to deal with a specific brand of shit, they have every right to just blanket block the ones they know of that are huge annoyances. snow, jamie, luc, art, g... there's frequent names that crop up that are constantly in the midst of some kind of wank. i wouldn't be upset if a mod just put all the frequently named people on a blanket 'fuck off' list and kept it to a small group. but in answer to your question, it comes from experience with people they don't like, or being here long enough to be familiar with the troublemakers.