PunBB is released under The GNU General Public License version 2 or higher. In short terms this means that PunBB is free to download, use, distribute, modify and even charge for. However, if any of these modifications are released to the public, that code must also be released under the same license as PunBB.
You may remove the copyright from the footer, but please don't. If you must, please try to keep at least “Powered by PunBB” with the link to http://punbb.informer.com/ of some form. However the copyright notice in all the source files must be left intact. Any modification or removal of this copyright is illegal under the terms of the GNU General Public Licence.
Please, contact PunBB team if you have any questions or if you find some site breaking the PunBB copyright.