1. Stop redo to standby
2. Set flashback off on standby
3. Set flashback on on standby
4. Start redo apply to standby

On standby

idle>select * from v$flash_recovery_area_usage;

FILE_TYPE               PERCENT_SPACE_USED PERCENT_SPACE_RECLAIMABLE NUMBER_OF_FILES     CON_ID
----------------------- ------------------ ------------------------- --------------- ----------
CONTROL FILE                             0                         0               0          0
REDO LOG                                 0                         0               0          0
ARCHIVED LOG                           .16                         0               4          0
BACKUP PIECE                           .01                         0               1          0
IMAGE COPY                               0                         0               0          0
FLASHBACK LOG                        46.95                         0              64          0
FOREIGN ARCHIVED LOG                     0                         0               0          0
AUXILIARY DATAFILE COPY                  0                         0               0          0

On Primary:
Use dgmgrl:
DGMGRL> connect sys@ORCL
Password:
Connected as SYSDBA.
DGMGRL> edit database orclstby set state='apply-off';
Succeeded.

On standby:
idle>ALTER DATABASE FLASHBACK OFF;

Database altered.

idle>alter database flashback on;

Database altered.

idle>select * from v$flash_recovery_area_usage;

FILE_TYPE               PERCENT_SPACE_USED PERCENT_SPACE_RECLAIMABLE NUMBER_OF_FILES     CON_ID
----------------------- ------------------ ------------------------- --------------- ----------
CONTROL FILE                             0                         0               0          0
REDO LOG                                 0                         0               0          0
ARCHIVED LOG                           .22                         0               7          0
BACKUP PIECE                           .01                         0               1          0
IMAGE COPY                               0                         0               0          0
FLASHBACK LOG                          1.5                         0               2          0
FOREIGN ARCHIVED LOG                     0                         0               0          0
AUXILIARY DATAFILE COPY                  0                         0               0          0

On Primary:

DGMGRL> edit database orclstby set state='apply-on';
Succeeded.