MyBB Community Forums

can easily be bypassed if browser data is cleared. only way this will work is if the person doesn't know how to clear browser data, and there IP is banned

Use the Maximum Registrations Per IP Address in:

ACP > Configuration/Board Settings > User Registration and Profile Options

As other people have pointed out this plugin is pointless.

Even using the "Maximum Registrations Per IP Address" if someone is banned and wants to get back on the forum they will be able to do so with a proxy.
You could get around this by using a reverse Proxy Header that checks the header to locate the true IP address of the connecting client.

Something like:

'HTTP_X_FORWARDED_FOR',
'HTTP_FORWARDED_FOR',
'HTTP_X_FORWARDED',
'HTTP_FORWARDED',
'HTTP_CLIENT_IP',
'HTTP_FORWARDED_FOR_IP'
'X_FORWARDED_FOR',
'FORWARDED_FOR',

Or use one of these plugins.
http://community.mybb.com/thread-94101.html
http://community.mybb.com/thread-128639.html

But when you start blocking proxies you will start having other issues like legitimate users not being able to use the forum.

The safest way to block a proxy, you will need to have a list of know proxies to block. Maybe use the http://www.dronebl.org/ database.

But then again. How hard would it be to just use your phones 3G/4G/LTE network or a cafe/friends internet to make a new account?

Stopping Banned users from logging out so they can't make a new account just isn't going to work. The only true way to keep banned users banned is moderating your forum.