One of the most annoying things about phpBB is the ability for people to randomly use [size="200"]Huge[/size] BB code, with no easy way to remove it and every time you update between versions it comes back.
To disable it:
in /include/bbcode.php remove the whole
case 5: ... break;
section.
To remove it from the posting page:
in /styles/prosilver/template/posting_buttons.html remove the whole
<select> ... </select>
section.
After that, delete your “cache” directory so that it uses the new copy.
Remember that you have to repeat this process after every update to phpBB3.