Hilton thought they had tracked down some of the reservations that were causing the problem and told me that my email address was not associated with those. This doesn't explain how I received the reservations for these guests however. Hilton suggested it may be a problem with mailing lists at the hotel in question, although the mails are coming from Hilton centrally and not from the property. Hilton now seem to have abandoned their investigations so I have no idea what happened, and have no confidence that the problem won't recur. Hopeless.