The macros below can be used to delete all items in the Junk and Trash (Deleted Items) folder in Outlook.
For ease of use, attach the first macro to a button in the Quick Access Toolbar:

Sub DeleteAllSpamAndTrash()
' updated 2017-01-10 by OPE
Dim OLF As Outlook.NameSpace, f As Long, objFolder As Folder
    Set OLF = GetObject("", "Outlook.Application").GetNamespace("MAPI")
    For f = 1 To OLF.Folders.Count
        On Error Resume Next
        Set objFolder = OLF.Folders(f).Store.GetDefaultFolder(olFolderJunk)
        On Error GoTo 0
        DeleteAllFolderItems objFolder
        Set objFolder = Nothing
        
        On Error Resume Next
        Set objFolder = OLF.Folders(f).Store.GetDefaultFolder(olFolderDeletedItems)
        On Error GoTo 0
        DeleteAllFolderItems objFolder
        Set objFolder = Nothing
    Next f
    Set OLF = Nothing
End Sub

Sub DeleteAllFolderItems(objFolder As Folder)
' updated 2017-01-10 by OPE
Dim i As Long
    If objFolder Is Nothing Then Exit Sub
    
    With objFolder
        For i = .Items.Count To 1 Step -1
            On Error Resume Next
            .Items(i).Delete
            On Error GoTo 0
        Next i
    End With
End Sub