In Enterprise Manager 12c SYSMAN user is the schema owner and as a best practice all the users should log in using their own individual accounts. To enforce this you can prevent SYSMAN from login into the console and/or emcli by setting SYSTEM_USER to -1 in the MGMT_CREATED_USERS table:
To re-enable the access just set it to 1.
Refer to Oracle Support’s note:
How To Disable SYSMAN & SYSTEM Users from Logging into Grid Console? (Doc ID 867360.1)