How to stop streaming replication of PostgreSQL DB?

Do you mean to stop replication of RDS in a multi-AZ deployment?
A multi-AZ configuration is a setup that uses replication to maintain redundant and consistent standby copies of data.
In other words, I believe that replication settings for this part of the system are managed by AWS and cannot be changed.
https://repost.aws/knowledge-center/rds-convert-single-az-multi-az

Replication also cannot be stopped for multi-AZ clusters.
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_MultiAZDBCluster_ReadRepl.html#multi-az-db-clusters-create-instance-read-replica