Not so that they can't post or anything, so that they can't even do anything at all.
This would still allow unbanned IPs to create new accounts for family members, but when an IP is banned, that means that the only other accounts coming from there are duplicate accounts meant to cause problems.
This should already be the case. The problem with Mr Shadow is he's managing to come back on new IP addresses each time. It's quite rare for IPs to change so much...