You can also edit your inc/toolbar.php file (note you'll want to save a backup of the original, and you'll need to patch this in again every time dokuwiki is updated). The new code will look something like the following below.
As part of such a modification, you'll also need to create an image file called unformat.png in lib\images\toolbar.
array(
'type' => 'format',
'title' => 'unformatted',
'icon' => 'unformat.png',
'key' => '5',
'open' => '%%',
'close' => '%%',
'block' => false
),