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The new changes have undoubtedly made a difference for the forum, but as you said, and many others have been saying, a requirement of more than 5 Merits to make the signatures available would have made the difference much more bigger than this. But, as they say, something is better than nothing.
you can filter and set some banned words to prevent spammers from commenting.
Filtered words won't prevent spammers from spamming, since they can always use different words if they come to know what words are being filtered. And it is not possible to stop this by filters since the words they use to spam are the words other users also use to post, so it won't just stop the spammers from posting but it would not allow others to use the same words as well. That solution won't work in this case.
Or perhaps a moderated new topic and/or comments? It will surely take a lot of work but maybe it will alleviate the spam in the discussion board.
That is what I suggested here, if you read the OP, you would have known. I asked people to create self-moderated threads in Bitcoin Discussion board to keep the spirit alive, and don't just let the spammers take over the board. A self-moderated thread can do good since the creator can easily remove useless comments and keep the discussion clean all the time.