I am a web developer at IGNOU. I have been maintaining IGNOU Website ignou.ac.in since August, 2013. Yesterday, my colleague found that there were number of users automatically registered and asked me to handle the issue.
I studied the issue and found that the users registered were spam having similar kind of email addresses. I explored the internet and found a solution.
Block the offending domains:
I went to the Access Rules section in the admin panel (admin/user/rules), and added a DENY rule for each of the domains. The rule configuration takes a wildcard, so I could simply enter %@http://pray.agencja-csk.pl.
I also found that access rules functionality has been removed from the Drupal 7 core. It is now provided by the User restrictions module.