A list of substances that have been tested for inhibition of Mn-SOD can be found here:

http://brenda-enzymes.org/enzyme.php?ecno=1.15.1.1#INHIBITORS

Most of the things on the list are obviously not specific.

A shorter list of inhibitors of human Mn-SOD can be found here:

http://brenda-enzymes.org/enzyme.php?ecno=1.15.1.1&Suchword=&reference=&UniProtAcc=&organism%5B%5D=Homo+sapiens&show_tm=0#INHIBITORS