A bit of an odd one here (and has been reported to Ruckus as something that either needs clarifying or fixing) in that if you want to remove the requirement for CAPTCHA when self registering in a ‘out of band’ workflow block you need to configure your own SMTP server. Now that’s all fair however even with the server configured the disable option still doesn’t appear.

In this case it looks like you also need to disable (or configure yourself) the built in SMS server at Administration > System Service > Outbound SMS. With that set the disable option then appears in the drop down.