dear you can take help from the face book team after telling them your problem you will get the answer about this i have also faced this problem but i do not bother with this.
There are so many messages in my FB account message box, including lots of spam messages, that caused I often miss some useful message. I tried to delete all of them, but it seems only can delete them one by one, can't find the "Delete all the messages" button any more, it's really impossible to select and delete all the spam message one by one. Does facebook delete the function or do I miss something?
Thanks for your guide.
Well I looked through my message box on FB too and can't find a way to delete multiple messages either. Looks like only individual messages from people, or full conversations. I tried anything I could think of but couldn't come up with anything. Please do update if you find a way to delete multiple messages at once.