1. change the Cycle property of the form to "current record". That will prevent the tab from moving to a new record.
2. If you want to control the saving of a record, you must put your code into the FORM's BeforeUpdate event. That event accepts a cancel argument which will allow you to cancel the save if you determine that you don't want the user to save the record. You could even go so far as to "undo" his changes although, I recommend against this unless the user ok's the option.