I'm pretty sure you can't stop the edit session from within an editor event like this. A typical workflow would be for the user to start the edit session, sketch a new feature, you listen for the OnCreateFeature event and pop up your dialog which would in turn populate attributes on the new feature and close. The user would then continue their edit session. If they save, then your attribute edits would be saved along with the new feature. If they cancel, then your attribute edits are discarded. They could also continue creating more features and your dialog would pop up for each. If want a different workflow, then you may find it easier to skip using the editor altogether and simply write a tool that does what you need it to do.