Maybe you have a magical mystery glitch?
The ability to Archive an item only appears when viewing items in your Purchase History. When you tick the box to the left of an item and choose the Archive button, the item is removed from your Recent Purchase History (which covers the past 60 days) and you are given the message:
"Item(s) successfully hidden. Go to “Archived” in Purchase History to see them again."
Archive is not offered for items in your Watch List or Bids/Offers list. If you are seeing it there sometimes, then it is an error. Which would explain why the items reappear. The only way to permanently remove items from your Watch list is to tick the box for the item and choose Delete. You cannot remove items from your Bid/Offers list (they disappear when the auction ends).