Whereas I have enabled setting "Retain the post for manual moderation" for my wiki node type form, and whereas this setting evidently keeps unpublished only a newly added node, but doesn't unpublish previously published and later edited node or its latest spammy revision - the spammers bypass the Mollom protection by initially adding a clean (non-spammy) node without any links or other text that would trigger Mollom's blocking mechanism, and afterwards re-editing this clean node into a very spammy node, but Mollom neither unpublishes the node, nor this particular spammy revision.

Sufficient and simple solution would be just auto unpublishing the whole node, but a more brilliant solution would be something similar to http://drupal.org/project/revisioning module - unpublishing only the spammy revision, while keeping active the previous non-spammy revision.