To remove Powered by phpBB® Forum Software © phpBB Group from your site. Remember by removing powerd by phpbb lines from your site you will not able to get help from their support forum. As this is free software they want this as help to them.
Login to your FTP or access your phpbb files from cpanel of phpbb software, go to directory styles usually default template is prosilver go to your template directory get file there overall_footer.html edit in any text editor and remove the powerd by phpbb text there. You are done.