If you receive mails inspite of blocking the sender, it may be because the sender is using multiple email ID's. You should try the following:
You can set a filter with obvious keywords that have been used in the subject of the emails. When you set the filter, Rediffmail system will divert the emails containing the mentioned keyword in their subject line to the specified folder.
You can select the dirty emails and click on the 'This is Spam' button to send them to the Junk Mail folder. The sender of the mail(s) will be automatically added to our list of suspected spammers and blocked from sending you any further mail.