One odd thing about pprune is that the rules are rarely if ever stated, and while we have the nicknames of the mods, when they act it is (in all cases I know of) anonymous. And sometimes one gets banned by the server owner; I got banned once and one of the mods (who I asked) was apparently unable to find out why.
This is how forums work. The owner of the server runs the show; it is his bouncy castle.
Different forums within pprune have different attitudes. I know people who got banned for pretty innocuous posts on some of the airline-related forums. I have also heard reports from years ago (no idea if true, and I doubt it) of people getting kicked off Private Flying for pushing the case for pistons twins.
You can set up your own forum; technically it is very easy these days. For the level of activity in the PF section here, a simple install of PHP-BB on an ADSL-connected PC would be totally adequate. But the moderation challenges will not go away; there will always be characters turning up who cause "trouble" and they will need to be kicked off. And, judging from some forums that have been out there for yonks and are very quiet, there won't be many visitors. Pprune did a lot of publicity over the years and a lot of people visit. Plus you will have to deal with the IT challenges resulting from kids hacking the off the shelf forum software; every pilot forum has been infected in recent times.
If you do get kicked off, you can always create another ID. You might have to knock up a fresh IP... or use a 3G IP or something like that. And start with a clean browser installation (no cookies etc).