How to disallow users with certain emails form posting? On a Drupal 6 site, I struggle with some persistent spammer who uses different tricks to escape any regexp spam filters. Whenever I delete him, he always uses certain email provider (e.g. somerandomname@blablamail.com) to register again. 

So I am wondering how can I disalow blablamail.com altogether so that either users can not register with this email provider, or better, they can not post anything after registration?
You can create an access rule and deny creating an account or logging in with an account that matches certain criteria. 

Go to admin/user/rules and add a new rule. In your case, the access type should be deny, rule type should be e-mail, and the mask can be '%@blablamail.com' for instance. You can test your new access rule by clicking on the Check rules tab afterwards.