Going by what other people have been banned for, there's quite a handful of unwritten rules. I don't think anyone's listed them before...so here I go.
Don't repost failbinned add-ons - Add-ons are failbinned for a reason. Usually they're broken and spammy. Reposting them is basically saying "Hey look at me, I draw with inkless pens!"
Do not change the title to a stupid or irrelevant title - If you're gonna change the title, at least make it relevant to the thread.
Acting too cool for the internet results in a ban - Do not go and say that somebody has no life, or you're above the community. This counts as a plea for attention.
Being too stupid to live can get you a ban - This goes hand-in-hand with the "think before you post" rule. If you can't post without making a complete idiot of yourself, chances are you don't belong on the internet.
Unnecessary or unprovoked flaming is a no-no - You'd better have a good reason for constantly flaming somebody. Over-doing it will also get you banned.
Do not admit to trolling - It's stupid to troll, but it's even dumber to outwardly say you're trolling. This is a well-justified ban reason. You can also be banned for admitting to trolling ingame.
Being a furry too much - It's actually possible to be so much of a furry to get banned. It may sound far-fetched, but that's an actual reason.
Posting or boasting about hacking can get you banned - Hacking ruins the fun for others in a game. Believe it or not, but boasting about hacking makes you look like a jerkwad.
Do not spam PMs or Reports - It counts as spam nonetheless, and doing this will get you banned, and have your ability to do so removed.
Constant begging will get you banned - Begging once in awhile is okay, but over-the-top diehard begging gets annoying, and gets you banned.
Overuse of BBC tags counts as spam - There is no need to use sixteen tags just for a sentence. There's also no need to resize your text any farther than 40.
Using gigantic images, or unnecessary page-stretching results in a ban - Believe it or not, nobody wants to see gigantic images stretching their screen and crashing their browser. It also is as equally as annoying when somebody page-stretches using other methods.
That's pretty much all the unwritten rules I know about. I hope these help.
