How do you delete an AWS EMR Cluster?

They will stay around for a while and will be automatically deleted later. There is apparently no fixed time documented anywhere.

They stay around for 60 days which is now documented here: https://docs.aws.amazon.com/emr/latest/ManagementGuide/emr-manage-view-clusters.html

Amazon EMR saves metadata about terminated clusters for your reference for two months, after which the metadata is deleted.