Only the admin theymos can turn a thread from regular to self mod after it has been made, but there needs to be a very good reason for that, and also you cannot act on posts made before the thread was converted as those users did not subscribe to be moderated by other users;
I can manually make threads self-moderated (only I can do this), but I only do this if the thread is extremely long/entrenched and the OP is trustworthy, since you are not allowed to delete any posts made before the topic became self-moderated.
The only way to turn off self mod would be the option suggested by tranthidung