In the past there have been forum threads that were removed (I presume by the author). To my knowledge those thread didn't get any answer before they were removed. I don't know if it's a good idea to be able to delete thread, even if there wasn't any answer (some times I wanted to answer but didn't had the time, or needed to look a bit more into it) but, in my opinion, it shouldn't be possible to remove a thread that has answers. If only because it's deleting what other people said. Maybe the original poster of a thread could lock it instead.
The only peoples that I think should be able to remove threads are staff members (to remove spam for example). If a non spam thread is removed, a link to it should still work but show a message "Thread was deleted by the staff because...".
As an example, a few days ago there was a thread about developing without a valid Windows License that had answers and that is now gone.
Is it a good idea to let people delete threads that have replies ?
After discussing this matter, the team and I have agreed that it is best to allow users to remove the threads that they have created. While we realize this can produce some occasional negative side effects like not allowing important information and lessons to remain persistently available, we feel that a user's freedom to control their own posts and, in general, their online presence is more important. At the end of the day, both possibilities include biting one bullet or another, but we've decided to stick with the original decision to allow users to delete their own threads.
This all being said, there is an avenue available for users to create persistently available information for the site: The Wiki. If you find a bit of information from a forum thread particularly valuable, I recommend creating a Wiki post explaining that information, so that the information can remain available to users of the site, but the connection between that information and a user's thread is severed, so that the thread creator can maintain control over their own posts.
Hopefully this is understandable; let me know if you have any additional questions.