Amazon ElastiCache
User Guide (API Version 2015-02-02)

Decreasing Read Capacity

To decrease read capacity, delete one or more read replicas from your Redis cluster with replicas (called replication group in the API/CLI). If the cluster is Multi-AZ with automatic failover enabled, you cannot delete the last read replica without first disabling Multi-AZ with automatic failover. For more information, see Modifying a Cluster with Replicas.

For more information, see Deleting a Read Replica.