Problems when trying to Undo edit operations in ArcGIS

According to the documentation here, you need to raise an error rather than using AbortEditOperation when you want to cancel an edit in OnDelete.

If you are using the OnCreate, OnDelete or OnChange methods in a class extension to validate edit operations, you should not call AbortEditOperation on the workspace if your logic indicates that the edit operation is invalid. Instead, raise an error which will be propagated to the application that is performing the edit on the class.