Typically not a permission granted to users as they can be abused to end a conversation prematurely. Plus I don't see the point to locking for 99% of scenarios. Why would anyone want to prevent discussion? Provide a valid use case (that can't be handled in a better way than locking the thread) and I will consider it.
In the past it's been offered to sponsors (on other websites). If you need something closed, feel free to hit one of us up.
