Before starting make sure you have a backup of your files.

Open posting.php

Find:

Code:
if (($mode == 'reply' || $mode == 'quote') && !$submit && !$preview && !$refresh)

{

   $post_data['post_subject'] = ((strpos($post_data['post_subject'], 'Re: ') !== 0) ? 'Re: ' : '') . censor_text($post_data['post_subject']);

}
Replace with:
Code:
if (($mode == 'reply' || $mode == 'quote') && !$submit && !$preview && !$refresh)

{

   $post_data['post_subject'] = ((strpos($post_data['post_subject'], 'Re: ') !== 0) ? '' : '') . censor_text($post_data['post_subject']);

}
Save and replace your files.