Aye? You're kidding right?
You'd ban people from using webmail accounts...that's a bit silly, not everyone can afford to set up their own ISP account. Some people might share an internet connection (dial up) at home and use webmail to keep their email seperate from the account holder's. Multiple email boxes or not, it's a silly move.
I use a webmail account because I don't want non-work related emails coming to me at work.
Bit OTT IMHO.
Keep the current structure but give any offender 3 warnings to clean up their act. Naturally this is up to mods as it is their unenviable job to ensure a basic level of humane discussion.
With the right moderating team, any problems can be solved on the boards.
Sure you will get some dropkicks who insist on coming back - but they're easily dealt with the same way as they were before. And eventually they'll either wise up and stop being fools, or they'll continue to be lame fools.
Either way problem solved.
Don't forget with the growth of ADSL it's possible to ban via IP address now too.