I had another undeletable Outlook folder, and all my tricks for cajoling Outlook into letting me delete it failed. I already deleted all the items in it, emptied it of offline items, deleted all the failures from my Sync Issues folder, but still I couldn't delete the folder.

But I came up with a new trick.

I moved the folder out of its normal location and made it a direct child of "Mailbox". As a result, it was no longer part of the list of synchronzized folders. I synchronized one more time, and then Outlook let me delete the folder.

As I noted before, I don't know if this'll work for you, but it's another trick you can add to your bag.

(And I still have three folders that I can't delete, despite trying both this trick and the previous one.)