Hi Magma,

[…] how I can prevent a person changing the Subject title when replying to a thread?

I think that would not be a particularly good idea. On the contrary, in most fora admins struggle to educate users to modify the subject line according to the post’s content. Quite commonly you observe the so-called “thread-drift”. In some of the replies the discussed topic might move away from the original one. If the subject line is modified it greatly helps others users not to be forced to browse through all posts – even uninteresting ones – and concentrate on the ones dealing with the OT only.

Therefore all forum-scripts I’m aware of allow modifying the subject line. It should be possible that you restrict that in your installation but I don’t expect to see such a “solution” posted by one of the regulars here…