That usually means that the listbox is using the Fuzzy Search mode by default. You cannot disable it in the document, but you can set document wise that the default search mode is Normal or Wildcard. Go to the Document Properties in the Settings menu, Presentation, and select the fields and the default search mode there.
Hope that helps.
EDIT: In the listbox properties, General, "More Search Settings" button, check the "List of Fields" radio button and leave the list empty, so when the fuzzy search is selected, QlikView will only search in that specific field.