It's possible, and given that you're using PHP a decent-ish solution would be HTML Purifier. Assuming you can install it via PEAR (which is the most straightforward way to do it), your example would look like:
require_once 'HTMLPurifier.auto.php';
$config = HTMLPurifier_Config::createDefault();
$config->set('HTML', 'Allowed', ''); // Allow Nothing
$purifier = new HTMLPurifier($config);
return $purifier->purify($_REQUEST['field_name']);
You're going to want to remove most of the editor buttons and other options from CKeditor as well to prevent your users from adding a lot of formatting that won't survive filtration.
However:
Using the full CKEditor stack for spellcheck is overkill! Why not just use a standalone, jQuery spellchecker like this one?