I think that post deletion by the thread makers should be enabled. The reason why I think so is because if a use makes a thread and it eventually gets to spammy or messed up then eventually a Knight or an admin will come by and close it as it is his/her duty. But I think that if the thread maker notices the thread is spammy or otherwised not fitting, therefore if the user thinks it's going bad then he'she can delete it and save the staff trouble.
This is at least what I think.