this should be jsp.dspace-admin.eperson-main.LoginAs.off

but in any case I will prefer to remove completely these i18n messages (the next 3 too). Actually we don't translate message that go in the log files and the i18n keys will be confusing as it starts with jsp. but refers to something that is not used by a jsp.
Change to a simple text "Login As feature is disabled" the other text are good as in your Messages.properties proposal (please remember to cleanup this file after have moved this text inline in the code)