This is easy with VBA macro like this:
Sub ClearLastVerb() On Error Resume Next For Each Item In ActiveExplorer.Selection Item.PropertyAccessor.SetProperty "http://schemas.microsoft.com/mapi/proptag/0x10800003", -1 Item.PropertyAccessor.SetProperty "http://schemas.microsoft.com/mapi/proptag/0x10810003", 0 Item.Save Next End Sub
Just add this macro to your Outlook and after that you can place the button that will clear icon and last verb executed for the selected messages.