The delete button should delete the email.
Archiving can easily be done by moving the email to the All Mail folder (that's how I used to do it using IMAP and it was great).

Archiving junk email is simply unusable. I can't even understand why there is this option using IMAP. By the way you should also change the default for IMAP to delete and not archive !

Please make this change. It is urgent and should not take you too long I guess.
I actually would be ready to pay to get this fixed.