It's only done by the site admins.
Lock a Forum

I just discovered (learnt from staff)... The person who has started a new topic/thread can delete the entire topic/thread by simply deleting the first posting... I have tried it and it works. If you ask a thread to be locked, you might simply be conveyed the mentioned information. It is upto the discretion of the staff whether they want to lock a topic/thread or not - simply requesting does not always get it locked.
Hey does anybody out there know how to lock a forum???