The spam filter blocks individual searches only. For instance, if you have "mpg" in you spam block list, Gnotella will not propogate any search with that exact keyword, no matter where that search originates.

What you are asking about is IP or host filtering, where you can block any search results that come from a specific place (like 'www.flatplanet.net').

Gnotella does not yet feature IP or host filtering. But I think shaun has this feature planned for a future release.

In summary, the spam blocker blocks search requests, while an IP blocker blocks search results from a specific IP (it would, in reality also block search requests, I think, but that is only secondary).