Jason Clark posted a Perl script called scrub. It is designed to delete comment spam from "standard" writeback comment files.

scrub is designed to work with comment files created by the writeback plugin. These files contain each comment, along with the name and url of the poster as supplied on the posting form. I've modified my copy of writeback to also log the IP address. Any information in the comment file can be matched by the regex... so if you are logging IPs as I am, you can quickly find (and eliminate) all comments from a given IP. scrub overrides perl's $/ magic variable, which is the input separator. By setting $/ to "-----\n" (the comment separator in writeback files), scrub can process each comment as a single unit.

You can read the specific documentation and download the script from Jason's site here (local download here).