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To stop the billing you can delete the cluster.
How to delete the cluster
Navigate your browser to HiveMQ Cloud and log in.
In the menu on the left-hand side of the page click on the cluster you want to check.
Scroll down the page to the section Danger Zone and click on “Delete Cluster”.
Will my settings be saved after I delete the cluster?
No settings, such as Assess Credentials or hostname will be saved after you delete the cluster.