I have been trying for an hour to find a checkbox or something at phpbb admin to turn off guest posting but couldn't. where is it?
You could find it in forum permissions. (AdminCP -> forum management -> permissions). You must set the new topic and reply level not to 'guest' but to 'registred'/user. Hope this helps, if not let me know.
Guests can't make posts if they are not registered (at least not in phpbb3) You do realize how old this thread is?