Hello, @Vij,
Welcome to the Microsoft Q&A platform!
Based on your description, I understand that you want to know how to remove the emails which are not quarantined by the Defender from the users email boxes. In order to provide you with a more effective solution, I would like to confirm your specific needs first.
If you want to quarantine the email from user mailbox based on subject, you could try to create a mail flow rule and set the conditions to meet what your need.
For more information, please refer to this article: Manage mail flow rules in Exchange Online | Microsoft Learn
Looking forward to your reply. Thank you for your support and understanding.
Best,
Alex Zhang