[Lynne]

You asked if it was possible and I said not with default vbulletin.

Anyway, you'd probably want to write a plugin that does a quick check to count posts in a thread. If there is more than 1 post, give them a No Permission for deleting the thread. I think you'd do this for the editpost.php file but I'm not positive (look at the form action for the Edit Post).

Programming Discussions is for help writing the code (you write it, tell us what is wrong, we make suggestions on how to fix it). If you need someone else to write it for you, you should post in Modifications Requestions (unpaid) or Paid Requests.