Why do I see many records with the name DELETE*** in front of them?
When record are merged in the DemandTools you can have the non-master records (the ones that are merged from) either deleted or prefixed with "DELETE***".
The default is to have the records deleted. You can change this directly in Single Table De-Dupe in the Control Options on the right hand side of the found matches screen, under "Non-Master Control". You can also customize which field is prefixed and what to prefix with (i.e, create a custom field in SF.com called "Marked for Deletion", and enter a "Y" in this field).
To remove the existing records that start with DELETE*** you can right click on the object in MassEffect and export just the accounts/records that start with "DELETE***" and then use MassEffect to delete them.