Are there any exceptional cases in which a person with O negative blood group cannot donate?
There are some situations, where, depending on the interpretation of Rh status, an exception could become possible. Rh +/- commonly refers to the D antigen. However, the Rh group has many antigens in it, including C and E. One can become sensitized to these other antigens and develop a hemolytic response to incompatible C or E type.

Dean L. Blood Groups and Red Cell Antigens [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2005. Chapter 7, The Rh blood group.