Sure, you can remove the credit line. You may want to take this rule into consideration first though:
The Rules wrote:
All users of the phpBB software are requested to retain the short message giving credit to phpBB (as it is displayed in the footer of a default installation
). If this is not possible for a justifiable reason, we ask that at least "Powered by phpBB
", with "phpBB" linked to "http://www.phpbb.com
message or image may be substituted, although the link must be preserved. Members who fail to consistently display a valid credit line will not receive support from this community.
In short, support will not be provided at phpBB.com for users who have removed the phpBB credit line from their forum.