They are not hackers.. they are just kids that found a vulnerability for phpBB and then searched for not updated sites (that still have the vulnerability).
Although they call themselves hackers, this is not hacking, it is defacing.. and as it is very easy to find the necessary exploits to perfom this type of attacks to some sites, they just choose a group of sites randomly from google and attack them.
Just update your phpBB version (and also all the mods you are using) and we are not going to see this people again in a long time
(if you keep up to date the site :P)
Good luck!