Some people have static IP addresses at home, meaning you can't change it.
As for changing the MAC address, it is hard coded into your phone. There are programs to spoof it, but the MAC address isn't visible to internet sites.
Bottom line the only way to "change" your IP is to use a proxy server. If we're talking about a website ban, try visting the site via http://anonymouse.org/
But: banning usually works by looking at more than just the IP address, other used data is OS, resolution, browser, etc.
You would have to reinstall the OS, requiring you to wipe all data from the phone in the process. This is only possible on Android, maybe windows phones too. Point being its probably not worth it. But if you still think its worth it, google "factory flash *your phone model*" and you will find something to reinstall your OS.
An awful lot of work just to evade a forum ban.