How to disable delete once the thread has been started in advanced forum module in drupal 7? If there are no comments on the current topic then user can delete the post.

If there are one or more comments on the forum-topic then user will not be able to edit or delete forum-topic.

Which module should i use or which hook needs to be implemented?
You're looking for hook_node_access() where $op == 'delete'.

http://api.drupal.org/api/drupal/modules%21node%21node.api.php/function/hook_node_access/7