I'd only do a ban if it was a member who contributed to the community, otherwise I'd delete them and all of their posts...
Nothing is going to stop them from re-registering; they can use a different free email provider to skirt email bans and most isp's don't give people a "forever" static ip address (mine through Comcast are usually about a month tops) or they could use one of the millions of free web based proxies they could go through Google for... if you have a determined user you're ultimately screwed.
