To disable captcha, use the

mfoss — Thu, 19 Jan 2012 12:48:55 +0000

To disable captcha, use the value 'undefined' instead of removing the option.

Regarding "Not allowed", there are many reasons for the registration to be disallowed.
And it isn't possible to determine that from those logs.

ejabberd - Comments for "Configuring register_web_admin: &quot;Not allowed&quot; error"