When you start a server, you can use the -gk option to set the level of permissions required for users to stop the server with DBSTOP or STOP ENGINE.
The default level of permissions required is DBA, which requires the SERVER OPERATOR system privilege, but you can also set the value to ALL or NONE. If you set it to NONE, even the a user with SERVER OPERATOR system privileges cannot execute STOP ENGINE. In a production environment, only the DBA should be allowed to stop the database server.
Running stop_iq at the UNIX command line, or Shutdown on Windows platforms, still allows you to stop the server and databases on the machine where the server was started.