Pine is a UNIX command line mail application. So how do you delete all messages in a folder?
The combination is
; A A D
If after pressing ; you see
[Command ";" not defined for this screen. Use ? for help]
then you don't have a check in the "Enable aggregate command set" settings checkbox. To enable it, go to the main menu (M) -> Setup (S) -> Config (C) and scroll down to "Enable aggregate command set". Then press X (to check it), E (Exit). Now repeat the above.
In the meantime, if you found this article useful, feel free to buy me a cup of coffee below.