Is it possible with Phpbb to hide the posts from new users until a moderator review them ?
Just to prevent spamboots.
Also can the moderator reply them without making the thread public visible ?
Yes, to the first. If you set the New member post limit in Registration settings to a value greater than zero, that number of posts must first be approved. No, as to the second, but the Moderator can PM the user.
I can stop all your spam. I can upgrade or update your Board. PM or email me. (Paid support)