Those banned members are using what is called a Proxy. They are using random or major IP address. When you ban the IP address you could be banning universities, cities, regions, or even block a whole country. I actually never block IP addresses anymore, because the banned user will always find a way back to your website one way or another. You can block the whole world if you want, but that is not your goal.
Here are some modifications you can installed on your board which will help discourage banned members. Of course they can use different computers, setup email addresses, or use a proxy, but these modifications will slow them down.
vMail - Verify Mail before registration.
Cyb - Advanced Registration
Stop the Registration Bots
Multiple Account Detection & Prevention
Restrict Private Messages to Public if not Enough Posts
QUICK BAN + Quick Edit User from postbit for vB3.8 - Instantly Ban Spammers!