I have been reporting spam on Gmane by going to the web-interface for years. For years, I tell you, like a savage.
Finally I got my act together and configured Gnus, so I can report spam directly from there.
[Gmane] topic, I added these topic parameters:
((spam-process ((spam spam-use-gmane))) (spam-summary-exit-behavior 'move-none) (spam-use-hashcash nil) (spam-use-crm114 nil))
I am not sure that the last three are needed, but... better safe than sorry.