You have the option of operating the database in RESTRICTED-SESSION mode. This means that only users with a specific access privilege can log onto the database (usually the database administrators).
See Starting up the Database System.

Use the SAPDBA function User and Security ® Check RESTRICTED MODE to check which mode you are currently using.

When you exit SAPDBA, the program checks the database mode. If you have been working in RESTRICTED-SESSION mode, SAPDBA asks if you want to change the mode to NORMAL, to enable all users to log onto the database.