Go to the Maintain Report Sets panel and select the Report Set using option 1. Zero out the Days Report Remains in Archive fields. Run the command REPARCHIVE with the parameter "expired." Once the REPDLTMON job finishes, delete the set.
Go to the Report Archive History panel. Enter option 4 next to the set name to erase its history. Delete all occurrences of the set's history. Then go to the Maintain Report Sets panel and delete the set.
Note: Once it's deleted, Report History cannot be restored.