How to disallow node authors to change node content if comments were submitted for that node?
If coding this is an option, you should look at hook_node_prepare. 
As mentioned there on the first line:


  Act on a node object about to be shown on the add/edit form.


A code snippet idea that could do this for you:

function SOME_MODULE_node_prepare($node) {
  if (isset($node-&gt;nid) &amp;&amp; $node-&gt;type == 'YOUR_CONTENT_TYPE' &amp;&amp; $node-&gt;comment_count&gt; 0) {
    drupal_set_message('You cannot alter this node anymore', 'warning');
    drupal_goto('node/' . $node-&gt;nid);
  }
}


Good luck!