How to disallow node authors to change node content if comments were submitted for that node? On my site we have enquiries (nodes) and price offers (comments). So if there's at least one price offered (a comment submitted) the enquiry details (node) shouldn't be changed. Is there any way to do that?
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!