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
Search for:
Code:
<xf:option name="conversation_locked" checked="{{ ($draft.conversation_open OR $conversation.conversation_open) ? '' : 'checked' }}">
{{ phrase('lock_conversation_no_responses_will_be_allowed') }}
</xf:option>
Replace with:
Code:
<xf:if is="{{$xf.visitor.isMemberOf([3])}}">
<xf:option name="conversation_locked" checked="{{ ($draft.conversation_open OR $conversation.conversation_open) ? '' : 'checked' }}">
{{ phrase('lock_conversation_no_responses_will_be_allowed') }}
</xf:option>
</xf:if>