PHPBB3-16574 will remove support for flash in phpBB 4.0. 

In 3.3 admins can almost completely 'hide it' and prevent it's usage but there is one place where it still appears. 

ACP->Post settings has the option Allow use of [FLASH] BBCode tag in posts: (If disallowed the [FLASH] BBCode tag is disabled in posts). If set to No, when posting the flash button doesn't appear but underneath the Smilies it still shows the flash status, specifically [flash] is OFF  

I believe the fix is easy, in styles/prosilver/template/posting_editor.html, surround line 68 with 

<!-- IF S_BBCODE_FLASH -->

<!-- ENDIF -->