Generally the goal of moderation is not to have to ban people.
Typically a temp ban happens when you just accumulate enough concurrent points worth of infractions (10) to be automatically banned, and they expire when the points drop below that threshold (the default period for infractions is one month).
Actually banning outright a poster that has been here for going on 2 decades, even if they've only really been problematic for the last 2 years, is usually a final straw that has involved broad staff discussion, multiple chances and a "come to Jesus" discussion with the poster involved.
"Ban on sight" is usually what happens when posters are banned, take a break and return only to behave exactly the same. When they return for short periods after that, it's usually just because maintaining a ban on sight is difficult and labour intensive, and some trolls are incredibly persistent and long running.