There are a few ways we'd actually go as far as to permanently ban someone. It could be how many times they've been banned, and it seems apparent they're of no use to us, or they could personally threaten one of the staff, or call them out on stuff, or they could be seriously abusing bugs, but a permanent ban for bug abuse has to be on a special case.
I would consider someone coming on and saying our server sucks as being somewhat minor, but obviously it would have more importance than other offenses that players have done. There are many different levels of rule breaking that players would get punished for.
I hope that makes sense to you.