Very often search bots abuse forums' search features and cause excessive CPU load. Such overloads may violate the AUP / TOS of hosting providers.
In order to prevent such problems in PHPBB2 it is recommended to allow the search function only for registered users. Here is how this can be done:
Go to your forum's parent directory and open the file search.php
Find the line "init_userprefs($userdata);" and right after it add:
if ($userdata['user_id'] == ANONYMOUS)
Save the file and your PHPBB2 will be protected from anonymous searches including bots.