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Sometimes we do remove their posts (though it is time consuming to remove them one-by-one unless we nuke them) and I have removed a few of his but at the same time if I delete all his crappy ones and leave the good ones it could look like he's a good poster and nothing's wrong when we request a ban. He's banned anyway now so it's moot.
Let me try to keep it short.
punishment does work as it reduces spam for a while.Yes, its a constant battle. I think offtopic spam is less important to punish though. Asides from those the smear shit on the wall, let them chatter, for all I care let someone pay for the chatter and let most of their posts be chatter. I think its more important to keep the sections where discussions happen clean.
Are there other commonly used methods that might be applicable here?
Positive reinforcement. Theymos (or someone good in his name) runs a campaign for the board in addition to usual ads. Exceptional rates (compared to other campaigns) but very strict anti spam policy. This would create a positive example for others to follow or to work towards. This is already given in a sense by other well run and high paying campaigns. It is also not unheard of that exceptional and highly active posters can negotiate special deals.
Broken window theory. Post quality is lower because posts are not removed. The existance of low quality posts attracts lower quality posts. As you state abvoe this is currently a technical limitation, this will hopefully improve in the new incarnation of bitcointalk.
Smalltalk. Spam in a sense is a cultural aceptable thing in social conversations, people talk about the weather or other things that have no real importance all the time, but not in the middle of a debate. Its is thus part of the human nature to spam and this needs an outlet, e.g. in the offtopic section. Smearing shit over the cubicle[1] is still not acceptable. Which puts us back to the problem that someone has to clean up the shit. Its not enough to boot the person if the toilet cant be used afterwards. It would however make more time to focus on forms of spam that are undisreable for discussions, e.g. in other sections.
The silent treatment. A silent ban or mute. The spammer can still post normally, but somehow everyone is ignoring them. The posts are not actually shown to anyone but the person with the silent ban. Probably a technical issue as I doubt it is already implemented. Its common for video games to keep players on the server or in the game without causing further issues due to chat/mic spam. Probably not worth it, considering that its easy to check with another account or with a simple logout.
No pocket money for a week. Let them post, but remove / hide the signature. No new suggestion, probably to difficult to implement for the little good it would do. At least for the current version of the forum software.
Gated community. Make the board invite only or otherwise drasticly limit new accounts. Get rid of the spammers. Will effectively lock out newbies, which is IMHO not worth the benefit.
In summary I guess punishment is all that is realistically possible. At least for now. I think the most important change with the new board would be that it will be easier to remove spammy posts en masse.
[1] I laughed out load at that analogy, I should have stayed with cookies