Why are some Requester conversations not deletable, (and how do I delete them :)

Why are some Requester conversations not deletable, (and how do I delete them :)

Hi,
 
For some reason, we often get items in the Requester Conversations list that don't need to be there, but which we are unable to delete.
 
This seems to have no pattern though. They might be locked, they might not. They might be from the requester, they might be from a technician, they might be from the system.
 
The bottom of a 'Conversation' that can be deleted looks like this:
 
 
 
 
 
 
The ones that can't just have 'Forward'
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
So two questions:
 
1. What determines whether a 'Delete' button gets displayed?
2. How can we get rid of those that don't?
 
Many thanks.

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