MyBB Community Forums

I just got amazed that a user can also edit their Own Thread Prefixe by editing his/her 1st post the thread.

Any suggestion or idea about it ?
how do i undone it or denied their permission that they will not be able to edit the thread prefix.

You can't, unless you forbid editing own posts or you use a plugin to avoid editing first post (not sure it exists)

you can peharps use https://community.mybb.com/mods.php?action=view&pid=33 to have an edit history of the post