How to disallow users with certain emails form posting?
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.