As mentioned earlier, Microsoft makes their money from Azure based on your usage. In order to avoid incurring costs, you should always clean up (that is, delete) resources that you no longer need. If, like me, you named your Cosmos DB instance bugtracker, then you can return to the Cosmos DB blade (you may have previously pinned this to your dashboard; otherwise, you should search for this) and select the instance.
Then simply select Delete Account:
As with many Azure resources, you have to complete a second step to confirm that you really want to delete the resource:
It may take a few seconds to complete, and after that, you're done.
