So, as a "responsible" ISP, you advocate denial of service attacks? You are either incredibly naive or intensely stupid to advocate that position. Is that how you want people to deal with you when .. ... .. .. have wiped the abuser out of your system. But in the event someone decides to mail the source to Linux 1000 times to your server, copying abuse, root, postmaster, and support, they kill off your entire site, denying thousands of innocent users Internet access. The number of hours I have wasted over the past four years chasing down hackers and mail bombers has been a real pain. I have ZERO tolerance for this behavior.
If your systems are so badly configured that a mail bomb attack denies your users access, then you don't qualify as a "responsible ISP" yourself. In fact, you qualify under both "naive" and "intensely stupid". I don't agree with mailbombing, but it sounds like you are ripping your clients off, since you obviously don't know to configure a system.