As you know, you can't prevent users from logging in via different IPs and Email addresses, but what I'd suggest is to use a combination of cookies and banning by email address. Also, I wouldn't reccomend letting 'em think they logged in but it just doesn't really work. That makes your site look broken when they figure out that their posts aren't going up, and may even invite more trouble.
Truly, I wouldn't be overly concerned about banned users switching to a new email address and creating a new account. You should log the IP of banned users and document the posts that break the usage agreement. Maybe a user gets banned, gets a new email address, clears their cookies and logs on and behaves responsibly from there on out. Fine. If they are constantly disruptive (especially if they are vulgar or threatening), an email or a phone call to their ISP may put a stop to it. You can't identify users by unique IP because they may be constantly changing, but the major ISPs know who had which IP address at what time. Yahoo is pretty good about canceling free accounts of users who abuse their service too.