Likely someone has entered a range of IPs to ban or else has used a wildcard resulting in multiple unintended IPs also being banned. Other than banning a specific troublesome member using IP bans in phpBB is best avoided.
You can specifically add that user's IP and use the Exclude from banning
option. However, if a range of IPs has been banned and the user, as for most people, has a dynamic IP address then they could run into the ban again as their IP address changes within the range allocated by their ISP.
If IP addresses are banned as ab anti-spam measure that is generally ineffective and their are far better methods, Preventing Spam in phpBB3