This simple template modification will remove registered users the possibility to start private conversations with the status Locked. I find it personally to be very annoying, as staff member, to receive questions or help requests which are (intentionally or not) locked, which also don't give me the possibility to reply, hence, to offer help.
So I came up with this simple template modification to remove for registered users the possibility to start a locked private conversation.
Please note they can still Lock the conversation after they create it, I didn't think it was necessary to completely remove for them the possibility to lock the conversation, so I just aim at preventing the creation of locked PCs, not the successive locking.
Here's what to do:
Admin Area->Appearence->Templates-> Open the template called conversation_add