Reject e-mail address from specific domain in simplenews newsletters e-mails
How are users registering their email addresses? For example, we use simplemail on http://goniseko.com in the footer. It's a signup block that anyone can use.  

Is this where your spammers are signing up from?

In my opinion, this is super easy to fix, if you aren't afraid of creating a quick and dirty module that uses hook_form_alter. 

In the module you would simply create a validation rule that if the form value submitted contained 'baddomainame.com' it would prevent the form from sending.

function mymodule_hook_form_alter(&amp;$form, &amp;$form_state, $form_id){
    if($form_id == 'my_simple_news_form_id'){
        $form[#validate][] = 'check_domain_callback_validate';
    }
}

function check_domain_callback_validate(&amp;$form, &amp;$form_state){
    if(&amp;$form_state['values']['simple-news-email-value'] == 'baddomainame.com'){
        form_set_error('', 'BAD DOMAINNAME, BAD! YOU NO SIGN UP');
    }
}


of course all of that is pseudo code, but the structure should get you started.  Here is a link to the forms API tutorial that will help if you are not very familiar with FAPI.

http://drupal.org/node/262422