Sorry about my English. I mean this, for example in ACP for guest the permissions 'send message' or 'viewing memberlist' is disabled, but if looks at viewonline.php Who is online
you can see
Guest Replying to message in 3.0.x Support Forum
Guest Replying to message in General Discussion
Guest Viewing user control panel
How Guest can Replying to message or Viewing user control panel? It's spambots.
Server can down, if spammers will be a lot of attack to phpBB3 via posting.php and memberlist.php
Need to disable SQL request in database (for guest) for stopping attack spammers via posting.php and memberlist.php of course, if it's possible.