In my experience there is no reliable way to ban someone - they can always get around it. For example, on Nicky's solution I've had several difficult users who just go and register under a different name.
If I ban by IP, then they use a different ISP. If I ban their email, they get another one.
What I would do is make sure you have email validation on (as this should slow them down a bit), then just keep banning them if they reregister (if they don't then you're lucky ). I once had a user who reregistered over 30 times in a week.
If you're having real major problems you could also temporarily disable registrations, with a message about why and an email address to send registration requests to - but only use this as a last resort as it's not really a good idea.
1) Create a new user group for your Banned members
2) Set permissions for this user group appropriately. They can't post etc.
3) Move the abusive member to this user group. Do not deleate the member.
Other thoughts: Consider moderating all new members. Consider banning all free email addresses and free domains for folks that wish to register. Admin has an optiin so this won't effect your current members.