Webform: Only Open Case if Condition is True
You can achieve this by:


Ensure you are using the latest version of webform_civicrm
(CiviCRM tab) Set the Case Type field to "user select"
(Webform tab) Edit the Case Type webform component, and change the widget type to "hidden". Set it's value to the case type of your choosing.
(Webform tab - Conditionals) Add a conditional rule to show/hide the Case Type element based on the value of another field of your choosing (it's a hidden field already, you say, what's the point of hiding it? Either way it won't appear to the user, but this conditional rule will prevent the case from being created if your condition is not met).