Two small changes to the htaccess-file controling the site made the world tick a little bit better:
First, I blocked the IP-range of a few US Internet Providers, then the regular expression about the WordPress site was changed. These things seemes to have slowed down / stopped the spammers for a while.
This site is not located in America, so there will not be to many problems about blocking the IP-ranges from that continent a while, or redirecting them to a static site, if that is what it takes.
