Longtime listener, first time poster (on this topic anyway)
This is NZNOG, not Newstalk ZB
BUT, and this is the important bit, Send an email to the MW developers telling them that as an ISP you are blocking customers that use their software in this way. Suggest that users of their software will find their Intarwebthingy disabled. Sure the developers may not like taking out neat features, but they probably won't like fielding calls from users who get their net killed because of their product either.
That would work - what would work just as well, if not better, is getting a news source to publish an article detailing the issue. A savvy head-line such as "Mailwasher gets users disabled" will work a treat. Nicky Nick will pull finger when he reads that in the technology section of the Herald or the Table of Contents in the Netguide. Only problem is persuading the right people - Who wants to sleep with Juha? ;-)