Answer
Add a check against the database's own identity to the Quick Report's query itself, using SYS_CONTEXT('UserEnv', 'Db_Name') — for example, filtering so the query returns zero rows entirely unless the database name matches your PROD instance's name. Rather than labeling the email by environment, this approach suppresses the report content outright from any non-PROD copy, so a stray TEST-environment run of the same scheduled quick report produces an empty result instead of a duplicate, confusingly-unlabeled report.