You need to set a global plugin error to stop it from CB Auto Actions. The below might work, but I have not tested such a scenario. Ideally you'd develop a plugin that adds new validation to fields like CB Query Field does with its query validation.
Code:
global $_PLUGINS;
$_PLUGINS->_setErrorMSG( 'Your registration failed!' );
$_PLUGINS->raiseError();
The above is basically how CB AntiSpam does it to stop blocked login/registration attempts.
Alternatively, is there a way to run custom PHP code as a validation rule against a specific field?
No, not yet, but a planned future plugin CB Code Field will provide this (it'll be like CB Query Field except with PHP).
krileon
