Cannot open Mail Preferences in Mountain Lion
Go to ~/Library/Containers/com.apple.mail/Data/Library/Preferences and move the file com.apple.mail.plist to the Desktop and restart Mail.

Other plist files exist in ~/Library/Mail/V2/MailData/ but I'd start with the above plist first. In this directory you could also manually remove the offending account (particularly if the mail is also stored online) and re-add it.