By using the clients pop3 isp email for registration, you eliminate all these 3rd party email accounts, and it tells you who the person's isp is, and makes it much easier to track them down should the need arise.
Also, it stops multiple account creation. It can be done here at FR I would think, and would put a stop to these lefty raids.
Ahh ... I think I see what you are proposing. No "free" e-mail addy can be used to register. No hotmail, hotpop, yahoo, gmail, etc. e-mail addy's are acceptable.
Yes, that would narrow the field. Only people who have an account with an ISP could register.
Also, it stops multiple account creation.
And would stop the husband/wife tag-teams here too ;-) Just kidding.
For a reviled public forum, FR has an amazingly small number of hit and run attacks.