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unfortunately not fully tested, as my hosts introduced PHPSuexec as a security measure and htaccess didn't work properly after that, I have to use php.ini.
as far as I can see it helped and there is alot you can do with htaccess to restrict spam, a search across the web, will show the htaccess route to stopping trackback and comment spam.
In the end I converted my whole site to the wordpress blog module, which is a pity, but works well enough.