Breck has given you the easy answer - use dbengX instead of dbsrvX - but you if want to continue to use the network server (and get full use of all CPUs), then you can start the server with:

dbsrvX -x tcpip{localonly=yes} ...other-switches-and-options...

If you just want to temporarily disallow network connections, then you can start your server normally and create a login procedure that checks the CommLink connection parameter.

Example:

SET OPTION public.disallow_non_local_connections = 'NO';

CREATE PROCEDURE DBA.login_check( )
BEGIN
      DECLARE INVALID_LOGON EXCEPTION FOR SQLSTATE '28000';
      // check if non-local connections are allowed
      IF CONNECTION_PROPERTY( 'CommLink' ) != 'local'
         AND ( select setting 
                 from sysoption 
                where "option" = 'disallow_non_local_connections' ) = 'YES'
      THEN
          SIGNAL INVALID_LOGON;
      ELSE
          CALL sp_login_environment;
      END IF;
END;

GRANT EXECUTE ON DBA.login_check TO PUBLIC;

SET OPTION public.login_procedure='DBA.login_check';

Then to disallow non-local connections temporarily, execute:

SET OPTION public.disallow_non_local_connections = 'YES';

And to re-enable non-local connecitons, execute:

SET OPTION public.disallow_non_local_connections = 'NO';