There are two ways to move messages between lists. If you are just moving one message at a time every message has quick action buttons on the right handside. The icons relate to the different lists available. Press the one you want and the message will move to the selected folder.
If your message board is just too popular you might want to do a bulk action and move lots of messages at one time. In this case just select the messages you want to move by pressing the check box next to each one. You might want to select all the messages in the list so in this case just press 'Select all'.
Now you'll see an arrow icon appear alongside all the different lists, just press one next to the list of your choice and the messages will move to the new list in an instant.
There is also a 'Delete' option so you can get rid of messages once and for all but be warned, if you delete it then it's gone forever. Maybe it's safest to store it in the hidden folder for now.