I made an extra page for wp-admin that lets you bulk delete comments based on key words in post_author or post_content. If anyone reading is competent with SQL and wordpress I'd be grateful if you'd have a look at the "despam" download at http://www.sweavo.34sp.com/downloads
have a play with it and let me know what you think, but back up your comments table first!
The code isn't conformant with the coding guidelines - I've only just become aware of them - but apart from that I'd appreciate any code review or user reviews. If anyone thinks it's feasible/worthwhile setting it up as a plugin, or grooming it for inclusion in the core code, let me know.