Never reply back to the sender of the Spam mail. Never even respond to the Spam email instructions to reply with the word 'remove'. You may only be confirming that your email ID is valid. It is best to delete the message and block the sender.
To do this, select email(s) you identify as Spam and click on the 'This is Spam' button. This will block the sender and report the email as Spam to the Rediffmail anti-spam system.
The anti-spam system takes reports like these from you and other Rediffmail users and blocks the incoming spam.