What to do? How to remove inactive/spam users?
1) You can prune (clean) them from PHP adminsitration manually. In administration Go to "Users & Groups" tab. Click "Prune Users".
Into "Last active:" field enter "0000-00-00". This will delete all users who registered an account but never loged in.
2) You can create php script with MYSQL search & delete command and setup cronjob to run this script regularly.
Script content might be: (but please backup your forum mysql first so you dont loose any important data) I dont guarantee these scripts are up to date and working.
<?php
// cron job to delete inactive users older than 48 hours
$db=mysql_connect('server','user','password');
mysql_select_db('your_phpbb_database_here',$db);
$strSQL="DELETE phpbb_users u, phpbb_user_group ug, " .
"phpbb_groups AS g FROM phpbb_users u, " .
"phpbb_user_group ug, phpbb_groups g WHERE " .
"u.user_active=0 AND u.user_id>0 AND " .
"u.user_id=ug.user_id AND ug.group_id=g.group_id " .
"AND g.group_single_user=1 AND " .
"FROM_UNIXTIME(u.user_regdate)<" .
"DATE_SUB(NOW(),INTERVAL 2 DAY);";
mysql_query($strSQL,$db) or die(mysql_error());
mysql_close($db);
?>
or
DELETE phpbb_users u, phpbb_user_group ug, phpbb_groups AS g FROM phpbb_users u, phpbb_user_group ug, phpbb_groups g WHERE u.user_active=0 AND u.user_id>0 AND u.user_id = ug.user_id AND ug.group_id = g.group_id AND g.group_single_user = 1 AND FROM_UNIXTIME(u.user_regdate)<DATE_SUB(NOW(),INTERVAL 2 DAY);
3) Install ADAP (Advanced Double Activation Pack PHPBB Mod) it has feature to auto prune junk not activated PHPBB users
https://www.phpbb.com/customise/db/mod/advanced_double_activation_pack/