To completely reset the data in your ChartMogul account, go to https://app.chartmogul.com/#admin/data-sources , click on your data connector e.g. Stripe and click Save & Re-Import Data. This will do a fresh import from your billing system into your ChartMogul account.
If you imported your data manually via a CSV import or through the user interface, you can delete customers individually by navigating to a customer's page in ChartMogul and clicking 'Delete'. To mass delete all of your manually imported customers please head to Admin > Data sources and click on "Delete all manual customers".