If I understand correctly this plugin event should halt the deletion if the plugin returns false according to Hugh's comment in this posting.
However, looking at the code in plugins/fabrik_form/php/php.php for onDeleteRowsForm, if the form php plugin returns false a JError::raisewarining is called and it is the resulting JExeption object that is returned rather than false so the test for a false fails in the runPlugins function and hence the record deletion proceeds.
I think this function needs to respond in the same manner as onBeforeProcess, onBeforeStore etc. as this is an onBefore type plugin call.
However, looking at the code in plugins/fabrik_form/php/php.php for onDeleteRowsForm, if the form php plugin returns false a JError::raisewarining is called and it is the resulting JExeption object that is returned rather than false so the test for a false fails in the runPlugins function and hence the record deletion proceeds.
I think this function needs to respond in the same manner as onBeforeProcess, onBeforeStore etc. as this is an onBefore type plugin call.