There aren't really any rules here as such, but please take note of these few points:
1. Respect other members; don't be a dick.
2. Try not to make annoying and/or pointless spam posts. Structured spam games are fine.
3. If you want a one-on-one conversation with someone, which is of no use or interest to anyone else, please take it to an instant messenger of your choice. Don't clog up the forums with it.
4. If you seriously can't control your spam urges, just do it in the No One Cares Bin. That way nobody has to pay attention to you.
5. If it's really necessary, you will be banned.
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Note: You are free to post whatever you like on the forums. However, if it contains nudity or any other graphic material, please link to it off-site - do not post the images here. This will prevent anyone wandering into a thread blissfully unaware that unsuitable images are about to appear on their screen.
You should mark the link NSFW (Not Safe For Work) if it's the kind of thing you wouldn't want to be looking at when your boss, teacher, parents or whoever are around.