My opinion is that you simply have a rule about it. We know who the main rapists are. We all know. So you simply have a rule, if you rape - ban. The regulars who rape main will already know the rule. So some new guy comes along, they are easy to shoot down. If the professional rapists would simply stop raping, we wouldn't need this discussion.
The HHK server addressed these issues. They had a rule at HHK, no main raping unless last flag. It wasn't a problem. Everybody knew the rule and if a new player came along and wanted to engage in main raping, there were warnings, admin kills, boot. It worked fine.
Other pluses at HHK:
They set up the Hastings map so that mains could not be taken.
They fixed the helo pads so that players could not stand on the pads. If you did, your health started down, like what happens when you go AWOL.
Great discussion on the topic but again, my pitch is simply change the rules. Some of us have Xfire. If a regular resorts to main raping when no admins are on, and I am on, I will record it. If it is recorded, it would be nice to see punishment handed down to the offending thugs.