SQL Server 2012 AlwaysOn Availability Group active/passive
You have to do two things 


ALTER AVAILABILITY GROUP group_name SECONDARY_ROLE ALLOW_CONNECTIONS =  NO 


The above will not allow connections to secondary. So your secondary will be TRUE standby and will be used when you do a failover.


ALTER AVAILABILITY GROUP group_name AUTOMATED_BACKUP_PREFERENCE = 'PRIMARY'


The above will indicate that your maintenance especially, backups will always happen on PRIMARY replica.

Since we are talking about maintenance, its worth mentioning the Recommendations for Index Maintenance with AlwaysOn Availability Groups