putting standby database into readonly

1. cancel standby recover mode.
SQL> alter database recover managed standby database cancel;

2. put db into readonly mode
SQL> recover standby database until cancel;

NOTE:
It is possible that the standby database was configured to shutdown the standby database instance when managed recovery mode is cancelled. If this is the case, the command above will shutdown the standby instance and you will need to mount the standby database in standby mode before attempting to put the standby database in READ ONLY mode:

SQL> startup nomount
SQL> alter database mount standby database;
SQL> recover standby database until cancel;